diff --git a/lib/Controller/PageController.php b/lib/Controller/PageController.php
index 0dbdfce034..640a9854ed 100644
--- a/lib/Controller/PageController.php
+++ b/lib/Controller/PageController.php
@@ -9,6 +9,7 @@
 
 use OC\App\CompareVersion;
 use OCA\Contacts\AppInfo\Application;
+use OCA\Contacts\Service\CustomPropertiesService;
 use OCA\Contacts\Service\GroupSharingService;
 use OCA\Contacts\Service\SocialApiService;
 use OCP\App\IAppManager;
@@ -33,6 +34,7 @@ public function __construct(
 		private IAppManager $appManager,
 		private CompareVersion $compareVersion,
 		private GroupSharingService $groupSharingService,
+		private CustomPropertiesService $customPropertiesService,
 	) {
 		parent::__construct(Application::APP_ID, $request);
 	}
@@ -75,6 +77,10 @@ public function index(): TemplateResponse {
 		$this->initialState->provideInitialState('isContactsInteractionEnabled', $isContactsInteractionEnabled);
 		$this->initialState->provideInitialState('isCirclesEnabled', $isCirclesEnabled && $isCircleVersionCompatible);
 		$this->initialState->provideInitialState('isTalkEnabled', $isTalkEnabled && $isTalkVersionCompatible);
+		$this->initialState->provideInitialState(
+			'customProperties',
+			$this->customPropertiesService->getCustomProperties(),
+		);
 
 		Util::addStyle(Application::APP_ID, 'contacts-main');
 		Util::addScript(Application::APP_ID, 'contacts-main');
diff --git a/lib/Service/CustomPropertiesService.php b/lib/Service/CustomPropertiesService.php
new file mode 100644
index 0000000000..945022ed1c
--- /dev/null
+++ b/lib/Service/CustomPropertiesService.php
@@ -0,0 +1,119 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace OCA\Contacts\Service;
+
+use OCA\Contacts\AppInfo\Application;
+use OCP\IConfig;
+use Psr\Log\LoggerInterface;
+
+/**
+ * Admin-defined custom property types for the contact editor.
+ *
+ * Configured as a JSON array in the app config:
+ *
+ * occ config:app:set contacts customProperties --value='[
+ *   {"name": "x-customernumber", "label": "Customer number"},
+ *   {"name": "x-region", "label": "Region", "force": "select",
+ *    "options": [{"id": "NORTH", "name": "North"}, {"id": "SOUTH", "name": "South"}]}
+ * ]'
+ *
+ * Supported keys per entry:
+ * - name (required): vCard property name, lowercase, must match /^x-[a-z0-9-]+$/
+ * - label (required): display name shown in the UI (not translated)
+ * - force: editor type, "text" (default) or "select"
+ * - options: list of {id, name} — TYPE parameter choices for "text",
+ *   value choices for "select" (required there)
+ * - multiple: whether the property may be added more than once per contact
+ * - primary: show on the first level of the "Add more info" menu
+ * - icon: CSS icon class, must match /^icon-[a-z0-9-]+$/
+ *
+ * Invalid entries are dropped and logged, they never break the app.
+ */
+class CustomPropertiesService {
+
+	public function __construct(
+		private IConfig $config,
+		private LoggerInterface $logger,
+	) {
+	}
+
+	public function getCustomProperties(): array {
+		$json = $this->config->getAppValue(Application::APP_ID, 'customProperties', '[]');
+		$decoded = json_decode($json, true);
+		if (!is_array($decoded)) {
+			$this->logger->warning('Ignoring contacts customProperties app config: not a JSON array');
+			return [];
+		}
+
+		$properties = [];
+		foreach ($decoded as $entry) {
+			$property = $this->sanitizeEntry($entry);
+			if ($property === null) {
+				$this->logger->warning('Ignoring invalid contacts customProperties entry', ['entry' => $entry]);
+				continue;
+			}
+			$properties[] = $property;
+		}
+		return $properties;
+	}
+
+	private function sanitizeEntry(mixed $entry): ?array {
+		if (!is_array($entry) || !is_string($entry['name'] ?? null) || !is_string($entry['label'] ?? null)) {
+			return null;
+		}
+
+		$name = strtolower($entry['name']);
+		$label = trim($entry['label']);
+		if ($label === '' || preg_match('/^x-[a-z0-9-]+$/', $name) !== 1) {
+			return null;
+		}
+
+		$force = $entry['force'] ?? 'text';
+		if (!in_array($force, ['text', 'select'], true)) {
+			return null;
+		}
+
+		$options = $this->sanitizeOptions($entry['options'] ?? []);
+		if ($force === 'select' && $options === []) {
+			return null;
+		}
+
+		$property = [
+			'name' => $name,
+			'label' => $label,
+			'force' => $force,
+			'multiple' => ($entry['multiple'] ?? false) === true,
+			'primary' => ($entry['primary'] ?? false) === true,
+		];
+		if ($options !== []) {
+			$property['options'] = $options;
+		}
+		if (is_string($entry['icon'] ?? null) && preg_match('/^icon-[a-z0-9-]+$/', $entry['icon']) === 1) {
+			$property['icon'] = $entry['icon'];
+		}
+		return $property;
+	}
+
+	private function sanitizeOptions(mixed $options): array {
+		if (!is_array($options)) {
+			return [];
+		}
+
+		$sanitized = [];
+		foreach ($options as $option) {
+			if (!is_array($option) || !is_string($option['id'] ?? null) || !is_string($option['name'] ?? null)
+				|| trim($option['id']) === '' || trim($option['name']) === '') {
+				continue;
+			}
+			$sanitized[] = ['id' => $option['id'], 'name' => $option['name']];
+		}
+		return $sanitized;
+	}
+}
diff --git a/src/models/customProperties.ts b/src/models/customProperties.ts
new file mode 100644
index 0000000000..8f5639eb65
--- /dev/null
+++ b/src/models/customProperties.ts
@@ -0,0 +1,70 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+interface PropertyTypeOption {
+	id: string
+	name: string
+}
+
+export interface CustomPropertyConfig {
+	name: string
+	label: string
+	force?: 'text' | 'select'
+	options?: PropertyTypeOption[]
+	multiple?: boolean
+	primary?: boolean
+	icon?: string
+}
+
+interface CustomPropertyModel {
+	readableName: string
+	icon: string
+	force: 'text' | 'select'
+	multiple: boolean
+	primary: boolean
+	options?: PropertyTypeOption[]
+	defaultValue?: { value: string, type: string[] }
+}
+
+/**
+ * Merge admin-defined custom property types (app config "customProperties",
+ * validated server-side by CustomPropertiesService) into the property registry.
+ *
+ * @param properties the rfcProps.properties registry to extend
+ * @param fieldOrder the rfcProps.fieldOrder list to extend
+ * @param customProperties sanitized entries from the initial state
+ */
+export function applyCustomProperties(properties: Record<string, object>, fieldOrder: string[], customProperties: CustomPropertyConfig[]) {
+	if (!Array.isArray(customProperties)) {
+		return
+	}
+
+	customProperties.forEach((custom) => {
+		const name = typeof custom?.name === 'string' ? custom.name.toLowerCase() : ''
+		if (!name.startsWith('x-') || properties[name]) {
+			return
+		}
+
+		const model: CustomPropertyModel = {
+			readableName: custom.label,
+			icon: custom.icon || 'icon-detailed-name',
+			force: custom.force === 'select' ? 'select' : 'text',
+			multiple: custom.multiple === true,
+			primary: custom.primary === true,
+		}
+
+		if (Array.isArray(custom.options) && custom.options.length > 0) {
+			model.options = custom.options
+			if (model.force === 'text') {
+				// options on a text property are TYPE choices: preselect the
+				// first one like the builtin tel/email/adr defaults do
+				model.defaultValue = { value: '', type: [custom.options[0].id] }
+			}
+		}
+
+		properties[name] = model
+		fieldOrder.push(name)
+	})
+}
diff --git a/src/models/rfcProps.js b/src/models/rfcProps.js
index 5acf08905f..475a3e3d0e 100644
--- a/src/models/rfcProps.js
+++ b/src/models/rfcProps.js
@@ -8,6 +8,7 @@ import ActionCopyNtoFN from '../components/Actions/ActionCopyNtoFN.vue'
 import NcActionToggleYear from '../components/Actions/NcActionToggleYear.vue'
 import logger from '../services/logger.js'
 import { otherContacts } from '../utils/chartUtils.js'
+import { applyCustomProperties } from './customProperties.ts'
 import zones from './zones.js'
 
 // Load the default profile (for example, home or work) configured by the user
@@ -433,4 +434,6 @@ const fieldOrder = [
 	'role',
 ]
 
+applyCustomProperties(properties, fieldOrder, loadState('contacts', 'customProperties', []))
+
 export default { properties, fieldOrder }
diff --git a/tests/javascript/models/customProperties.test.ts b/tests/javascript/models/customProperties.test.ts
new file mode 100644
index 0000000000..3d2009cd4a
--- /dev/null
+++ b/tests/javascript/models/customProperties.test.ts
@@ -0,0 +1,99 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import type { CustomPropertyConfig } from '../../../src/models/customProperties'
+import { applyCustomProperties } from '../../../src/models/customProperties'
+
+describe('customProperties', () => {
+
+	let properties: Record<string, object>
+	let fieldOrder: string[]
+
+	beforeEach(() => {
+		properties = {
+			tel: { readableName: 'Phone' },
+		}
+		fieldOrder = ['tel']
+	})
+
+	test('registers a plain text property with defaults', () => {
+		applyCustomProperties(properties, fieldOrder, [
+			{ name: 'x-customernumber', label: 'Customer number' },
+		])
+
+		expect(properties['x-customernumber']).toEqual({
+			readableName: 'Customer number',
+			icon: 'icon-detailed-name',
+			force: 'text',
+			multiple: false,
+			primary: false,
+		})
+		expect(fieldOrder).toContain('x-customernumber')
+	})
+
+	test('options on a text property become TYPE choices with the first one preselected', () => {
+		applyCustomProperties(properties, fieldOrder, [
+			{
+				name: 'x-office',
+				label: 'Office',
+				multiple: true,
+				options: [{ id: 'BERLIN', name: 'Berlin' }, { id: 'LINGEN', name: 'Lingen' }],
+			},
+		])
+
+		expect(properties['x-office']).toMatchObject({
+			multiple: true,
+			defaultValue: { value: '', type: ['BERLIN'] },
+		})
+	})
+
+	test('options on a select property are value choices without a preselected default', () => {
+		applyCustomProperties(properties, fieldOrder, [
+			{
+				name: 'x-region',
+				label: 'Region',
+				force: 'select',
+				options: [{ id: 'NORTH', name: 'North' }],
+			},
+		])
+
+		expect(properties['x-region']).toMatchObject({ force: 'select' })
+		expect(properties['x-region']).not.toHaveProperty('defaultValue')
+	})
+
+	test('skips names colliding with builtin properties', () => {
+		applyCustomProperties(properties, fieldOrder, [
+			{ name: 'tel', label: 'Phone override' },
+		])
+
+		expect(properties.tel).toEqual({ readableName: 'Phone' })
+		expect(fieldOrder).toEqual(['tel'])
+	})
+
+	test('skips entries without x- prefix or name', () => {
+		applyCustomProperties(properties, fieldOrder, [
+			{ name: 'customernumber', label: 'Customer number' },
+			{ label: 'No name' },
+			null,
+		] as CustomPropertyConfig[])
+
+		expect(Object.keys(properties)).toEqual(['tel'])
+	})
+
+	test('normalizes uppercase names', () => {
+		applyCustomProperties(properties, fieldOrder, [
+			{ name: 'X-Customernumber', label: 'Customer number' },
+		])
+
+		expect(properties['x-customernumber']).toBeDefined()
+	})
+
+	test('tolerates a non-array state', () => {
+		applyCustomProperties(properties, fieldOrder, undefined as unknown as CustomPropertyConfig[])
+		applyCustomProperties(properties, fieldOrder, 'garbage' as unknown as CustomPropertyConfig[])
+
+		expect(Object.keys(properties)).toEqual(['tel'])
+	})
+})
diff --git a/tests/unit/Controller/PageControllerTest.php b/tests/unit/Controller/PageControllerTest.php
index dfa30b4be3..f64daefb3f 100644
--- a/tests/unit/Controller/PageControllerTest.php
+++ b/tests/unit/Controller/PageControllerTest.php
@@ -9,6 +9,7 @@
 
 use ChristophWurst\Nextcloud\Testing\TestCase;
 use OC\App\CompareVersion;
+use OCA\Contacts\Service\CustomPropertiesService;
 use OCA\Contacts\Service\GroupSharingService;
 use OCA\Contacts\Service\SocialApiService;
 use OCP\App\IAppManager;
@@ -50,6 +51,8 @@ class PageControllerTest extends TestCase {
 
 	private GroupSharingService|MockObject $groupSharingService;
 
+	private CustomPropertiesService|MockObject $customPropertiesService;
+
 	protected function setUp(): void {
 		parent::setUp();
 
@@ -62,6 +65,7 @@ protected function setUp(): void {
 		$this->appManager = $this->createMock(IAppManager::class);
 		$this->compareVersion = $this->createMock(CompareVersion::class);
 		$this->groupSharingService = $this->createMock(GroupSharingService::class);
+		$this->customPropertiesService = $this->createMock(CustomPropertiesService::class);
 
 		$this->controller = new PageController(
 			$this->request,
@@ -73,6 +77,7 @@ protected function setUp(): void {
 			$this->appManager,
 			$this->compareVersion,
 			$this->groupSharingService,
+			$this->customPropertiesService,
 		);
 	}
 
diff --git a/tests/unit/Service/CustomPropertiesServiceTest.php b/tests/unit/Service/CustomPropertiesServiceTest.php
new file mode 100644
index 0000000000..21447f71e1
--- /dev/null
+++ b/tests/unit/Service/CustomPropertiesServiceTest.php
@@ -0,0 +1,131 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace unit\Service;
+
+use ChristophWurst\Nextcloud\Testing\TestCase;
+use OCA\Contacts\Service\CustomPropertiesService;
+use OCP\IConfig;
+use PHPUnit\Framework\MockObject\MockObject;
+use Psr\Log\LoggerInterface;
+
+class CustomPropertiesServiceTest extends TestCase {
+	private CustomPropertiesService $service;
+	private IConfig|MockObject $config;
+	private LoggerInterface|MockObject $logger;
+
+	protected function setUp(): void {
+		parent::setUp();
+
+		$this->config = $this->createMock(IConfig::class);
+		$this->logger = $this->createMock(LoggerInterface::class);
+
+		$this->service = new CustomPropertiesService(
+			$this->config,
+			$this->logger,
+		);
+	}
+
+	private function mockConfigValue(string $json): void {
+		$this->config->method('getAppValue')
+			->with('contacts', 'customProperties', '[]')
+			->willReturn($json);
+	}
+
+	public function testEmptyByDefault(): void {
+		$this->mockConfigValue('[]');
+
+		$this->assertSame([], $this->service->getCustomProperties());
+	}
+
+	public function testInvalidJsonIsIgnoredAndLogged(): void {
+		$this->mockConfigValue('{not json');
+		$this->logger->expects($this->once())->method('warning');
+
+		$this->assertSame([], $this->service->getCustomProperties());
+	}
+
+	public function testValidEntryIsNormalized(): void {
+		$this->mockConfigValue(json_encode([
+			[
+				'name' => 'X-Customernumber',
+				'label' => 'Customer number',
+				'icon' => 'icon-detailed-name',
+				'unknownKey' => 'is stripped',
+			],
+		]));
+
+		$this->assertSame([
+			[
+				'name' => 'x-customernumber',
+				'label' => 'Customer number',
+				'force' => 'text',
+				'multiple' => false,
+				'primary' => false,
+				'icon' => 'icon-detailed-name',
+			],
+		], $this->service->getCustomProperties());
+	}
+
+	public function testOptionsAreSanitized(): void {
+		$this->mockConfigValue(json_encode([
+			[
+				'name' => 'x-region',
+				'label' => 'Region',
+				'force' => 'select',
+				'multiple' => true,
+				'options' => [
+					['id' => 'NORTH', 'name' => 'North'],
+					['id' => '', 'name' => 'no id'],
+					['name' => 'missing id'],
+					'not an array',
+				],
+			],
+		]));
+
+		$properties = $this->service->getCustomProperties();
+		$this->assertCount(1, $properties);
+		$this->assertSame([['id' => 'NORTH', 'name' => 'North']], $properties[0]['options']);
+		$this->assertTrue($properties[0]['multiple']);
+	}
+
+	/**
+	 * @dataProvider provideInvalidEntries
+	 */
+	public function testInvalidEntriesAreDroppedAndLogged(mixed $entry): void {
+		$this->mockConfigValue(json_encode([$entry]));
+		$this->logger->expects($this->once())->method('warning');
+
+		$this->assertSame([], $this->service->getCustomProperties());
+	}
+
+	public static function provideInvalidEntries(): array {
+		return [
+			'not an array' => ['just a string'],
+			'missing name' => [['label' => 'No name']],
+			'missing label' => [['name' => 'x-foo']],
+			'empty label' => [['name' => 'x-foo', 'label' => '  ']],
+			'name without x- prefix' => [['name' => 'customernumber', 'label' => 'Customer number']],
+			'name with invalid characters' => [['name' => 'x-foo.bar', 'label' => 'Foo']],
+			'unknown force' => [['name' => 'x-foo', 'label' => 'Foo', 'force' => 'date']],
+			'select without options' => [['name' => 'x-foo', 'label' => 'Foo', 'force' => 'select']],
+		];
+	}
+
+	public function testValidEntriesSurviveInvalidSiblings(): void {
+		$this->mockConfigValue(json_encode([
+			['name' => 'x-valid', 'label' => 'Valid'],
+			['name' => 'invalid', 'label' => 'Invalid'],
+		]));
+
+		$properties = $this->service->getCustomProperties();
+		$this->assertCount(1, $properties);
+		$this->assertSame('x-valid', $properties[0]['name']);
+	}
+}