diff --git a/CHANGELOG.md b/CHANGELOG.md
index e7be24d421..6842713946 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,10 @@ Please add one entry in this file for each change in Yarn's behavior. Use the sa
 
 ## Master
 
+- Fixes `npm_config_` environment variable parsing to support those prefixed with underscore (ex: `_auth`)
+
+  [#7070](https://github.com/yarnpkg/yarn/pull/7070) - [**Nicholas Boll**](https://github.com/NicholasBoll)
+
 - Fixes yarn `upgrade --latest` for dependencies using `>` or `>=` range specifier
 
   [#7080](https://github.com/yarnpkg/yarn/pull/7080) - [**Xukai Wu**](https://github.com/shilcare)
diff --git a/__tests__/registries/npm-registry.js b/__tests__/registries/npm-registry.js
index 4848cef2db..b5177b7719 100644
--- a/__tests__/registries/npm-registry.js
+++ b/__tests__/registries/npm-registry.js
@@ -731,6 +731,37 @@ describe('isScopedPackage functional test', () => {
   });
 });
 
+describe('environment variables functional test', () => {
+  beforeEach(() => {
+    process.env.npm_config_always_auth = 'true';
+    process.env.npm_config__auth = 'auth';
+    process.env.npm_config__authtoken = 'authToken';
+    process.env.npm_config__username = 'username';
+    process.env.npm_config__password = 'password';
+  });
+
+  afterEach(() => {
+    delete process.env.npm_config_always_auth;
+    delete process.env.npm_config__auth;
+    delete process.env.npm_config__authToken;
+    delete process.env.npm_config__username;
+    delete process.env.npm_config__password;
+  });
+
+  test('correctly escapes environment config variables', () => {
+    const testCwd = '.';
+    const {mockRequestManager, mockRegistries, mockReporter} = createMocks();
+    const npmRegistry = new NpmRegistry(testCwd, mockRegistries, mockRequestManager, mockReporter, true, []);
+
+    npmRegistry.mergeEnv('npm_config_');
+    expect(npmRegistry.config).toHaveProperty('always-auth', true);
+    expect(npmRegistry.config).toHaveProperty('_auth', 'auth');
+    expect(npmRegistry.config).toHaveProperty('_authtoken', 'authToken');
+    expect(npmRegistry.config).toHaveProperty('_username', 'username');
+    expect(npmRegistry.config).toHaveProperty('_password', 'password');
+  });
+});
+
 describe('getRequestUrl functional test', () => {
   test('returns pathname when it is a full URL', () => {
     const testCwd = '.';
diff --git a/src/registries/base-registry.js b/src/registries/base-registry.js
index c563a7edfd..342362abcb 100644
--- a/src/registries/base-registry.js
+++ b/src/registries/base-registry.js
@@ -170,8 +170,8 @@ export default class BaseRegistry {
       // replace dunders with dots
       key = key.replace(/__/g, '.');
 
-      // replace underscores with dashes
-      key = key.replace(/_/g, '-');
+      // replace underscores with dashes ignoring keys that start with an underscore
+      key = key.replace(/([^_])_/g, '$1-');
 
       // set it via a path
       objectPath.set(this.config, key, val);