diff --git a/.chronus/changes/forward-typespec-npm-registry-2026-08-15.md b/.chronus/changes/forward-typespec-npm-registry-2026-08-15.md
new file mode 100644
index 00000000000..2152f7a454b
--- /dev/null
+++ b/.chronus/changes/forward-typespec-npm-registry-2026-08-15.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Forward `TYPESPEC_NPM_REGISTRY` to the npm process launched by `tsp install`.
diff --git a/packages/compiler/src/install/install.ts b/packages/compiler/src/install/install.ts
index 9460879ee66..ea2ee69e069 100644
--- a/packages/compiler/src/install/install.ts
+++ b/packages/compiler/src/install/install.ts
@@ -7,7 +7,11 @@ import { createDiagnosticCollector } from "../core/diagnostics.js";
 import { getDirectoryPath, joinPaths } from "../core/path-utils.js";
 import { NoTarget, type Diagnostic, type Tracer } from "../core/types.js";
 import { downloadAndExtractPackage } from "../package-manger/npm-package-download.js";
-import { fetchPackageManifest, type NpmManifest } from "../package-manger/npm-registry.js";
+import {
+  fetchPackageManifest,
+  getNpmRegistryEnvironment,
+  type NpmManifest,
+} from "../package-manger/npm-registry.js";
 import { mkTempDir } from "../utils/fs-utils.js";
 import type { SupportedPackageManager } from "./config.js";
 import { getPackageManagerConfig, type PackageManagerConfig } from "./config.js";
@@ -218,7 +222,7 @@ async function runPackageManager(
     stdio,
     cwd: directory,
     env: {
-      ...process.env,
+      ...getNpmRegistryEnvironment(),
       TYPESPEC_CLI_PASSTHROUGH: "1",
     },
   });
diff --git a/packages/compiler/src/package-manger/npm-registry.ts b/packages/compiler/src/package-manger/npm-registry.ts
index 4672eafe669..b12444aad62 100644
--- a/packages/compiler/src/package-manger/npm-registry.ts
+++ b/packages/compiler/src/package-manger/npm-registry.ts
@@ -90,7 +90,26 @@ const defaultRegistry = `https://registry.npmjs.org`;
  * otherwise falls back to the default npm registry.
  */
 export function getNpmRegistry(): string {
-  return (process.env["TYPESPEC_NPM_REGISTRY"] ?? defaultRegistry).replace(/\/$/, "");
+  const registry = process.env["TYPESPEC_NPM_REGISTRY"]?.trim();
+  return (registry || defaultRegistry).replace(/\/$/, "");
+}
+
+/** Returns the environment used to run npm with the TypeSpec registry override applied. */
+export function getNpmRegistryEnvironment(): NodeJS.ProcessEnv {
+  const environment = { ...process.env };
+  if (!process.env["TYPESPEC_NPM_REGISTRY"]?.trim()) {
+    return environment;
+  }
+
+  // Environment variable names are case-insensitive on Windows. Remove any
+  // existing spelling so Node does not choose between duplicate keys.
+  for (const name of Object.keys(environment)) {
+    if (name.toLowerCase() === "npm_config_registry") {
+      delete environment[name];
+    }
+  }
+  environment["npm_config_registry"] = getNpmRegistry();
+  return environment;
 }
 
 export async function fetchPackageManifest(
diff --git a/packages/compiler/test/package-manager/npm-registry.test.ts b/packages/compiler/test/package-manager/npm-registry.test.ts
index 02092ef5b98..f9c0edfaef9 100644
--- a/packages/compiler/test/package-manager/npm-registry.test.ts
+++ b/packages/compiler/test/package-manager/npm-registry.test.ts
@@ -1,7 +1,11 @@
 import * as http from "http";
 import type { AddressInfo } from "net";
 import { afterEach, beforeEach, expect, it } from "vitest";
-import { fetchPackageManifest } from "../../src/package-manger/npm-registry.js";
+import {
+  fetchPackageManifest,
+  getNpmRegistry,
+  getNpmRegistryEnvironment,
+} from "../../src/package-manger/npm-registry.js";
 
 let server: http.Server;
 let registryUrl: string;
@@ -34,6 +38,7 @@ beforeEach(async () => {
 
 afterEach(async () => {
   delete process.env["TYPESPEC_NPM_REGISTRY"];
+  delete process.env["NPM_CONFIG_REGISTRY"];
   await new Promise<void>((resolve) => server.close(() => resolve()));
 });
 
@@ -50,3 +55,33 @@ it("strips trailing slash from TYPESPEC_NPM_REGISTRY", async () => {
   expect(manifest.name).toBe("test-pkg");
   expect(lastRequestUrl).toBe("/test-pkg/1.0.0");
 });
+
+it("forwards TYPESPEC_NPM_REGISTRY to npm", () => {
+  process.env["TYPESPEC_NPM_REGISTRY"] = `${registryUrl}/`;
+  process.env["NPM_CONFIG_REGISTRY"] = "https://old-registry.example.com";
+
+  const environment = getNpmRegistryEnvironment();
+
+  expect(environment["npm_config_registry"]).toBe(registryUrl);
+  expect(environment).not.toHaveProperty("NPM_CONFIG_REGISTRY");
+});
+
+it("preserves the npm registry environment when no TypeSpec override is set", () => {
+  delete process.env["TYPESPEC_NPM_REGISTRY"];
+  process.env["NPM_CONFIG_REGISTRY"] = "https://configured-registry.example.com";
+
+  const environment = getNpmRegistryEnvironment();
+
+  expect(environment["NPM_CONFIG_REGISTRY"]).toBe("https://configured-registry.example.com");
+});
+
+it.each(["", "   "])("does not forward an empty TypeSpec registry override", (registry) => {
+  process.env["TYPESPEC_NPM_REGISTRY"] = registry;
+  process.env["NPM_CONFIG_REGISTRY"] = "https://configured-registry.example.com";
+
+  const environment = getNpmRegistryEnvironment();
+
+  expect(getNpmRegistry()).toBe("https://registry.npmjs.org");
+  expect(environment["NPM_CONFIG_REGISTRY"]).toBe("https://configured-registry.example.com");
+  expect(environment).not.toHaveProperty("npm_config_registry");
+});