diff --git a/sources/npmRegistryUtils.ts b/sources/npmRegistryUtils.ts
index a7d110e95..2666ccd96 100644
--- a/sources/npmRegistryUtils.ts
+++ b/sources/npmRegistryUtils.ts
@@ -14,7 +14,10 @@ export const DEFAULT_HEADERS: Record<string, string> = {
 export const DEFAULT_NPM_REGISTRY_URL = `https://registry.npmjs.org`;
 
 export async function fetchAsJson(packageName: string, version?: string) {
-  const npmRegistryUrl = process.env.COREPACK_NPM_REGISTRY || DEFAULT_NPM_REGISTRY_URL;
+  // Strip any trailing slashes so a `COREPACK_NPM_REGISTRY` with a trailing
+  // slash does not produce a double slash in the request URL (some registries,
+  // e.g. registry.npmmirror.com, reject `//package` with a 404).
+  const npmRegistryUrl = (process.env.COREPACK_NPM_REGISTRY || DEFAULT_NPM_REGISTRY_URL).replace(/\/+$/, ``);
 
   if (process.env.COREPACK_ENABLE_NETWORK === `0`)
     throw new UsageError(`Network access disabled by the environment; can't reach npm repository ${npmRegistryUrl}`);
diff --git a/tests/npmRegistryUtils.test.ts b/tests/npmRegistryUtils.test.ts
index 728d6f977..0ba9e8740 100644
--- a/tests/npmRegistryUtils.test.ts
+++ b/tests/npmRegistryUtils.test.ts
@@ -35,6 +35,15 @@ describe(`npm registry utils fetchAsJson`, () => {
     expect(httpFetchAsJson).lastCalledWith(`${process.env.COREPACK_NPM_REGISTRY}/package-name`, {headers: DEFAULT_HEADERS});
   });
 
+  it(`does not produce a double slash when COREPACK_NPM_REGISTRY has a trailing slash`, async () => {
+    // `process.env` is reset after each tests in setupTests.js.
+    process.env.COREPACK_NPM_REGISTRY = `https://registry.example.org/`;
+    await fetchAsJson(`package-name`);
+
+    expect(httpFetchAsJson).toBeCalled();
+    expect(httpFetchAsJson).lastCalledWith(`https://registry.example.org/package-name`, {headers: DEFAULT_HEADERS});
+  });
+
   it(`adds authorization header with bearer token if COREPACK_NPM_TOKEN is set`, async () => {
     // `process.env` is reset after each tests in setupTests.js.
     process.env.COREPACK_NPM_TOKEN = `foo`;