diff --git a/src/node_dotenv.cc b/src/node_dotenv.cc
index d8d6fc1d55d3de..ae18a6576dd232 100644
--- a/src/node_dotenv.cc
+++ b/src/node_dotenv.cc
@@ -48,14 +48,19 @@ void Dotenv::SetEnvironment(node::Environment* env) {
   for (const auto& entry : store_) {
     auto key = entry.first;
     auto value = entry.second;
-    env->env_vars()->Set(
-        isolate,
-        v8::String::NewFromUtf8(
-            isolate, key.data(), NewStringType::kNormal, key.size())
-            .ToLocalChecked(),
-        v8::String::NewFromUtf8(
-            isolate, value.data(), NewStringType::kNormal, value.size())
-            .ToLocalChecked());
+
+    auto existing = env->env_vars()->Get(key.data());
+
+    if (existing.IsNothing()) {
+      env->env_vars()->Set(
+          isolate,
+          v8::String::NewFromUtf8(
+              isolate, key.data(), NewStringType::kNormal, key.size())
+              .ToLocalChecked(),
+          v8::String::NewFromUtf8(
+              isolate, value.data(), NewStringType::kNormal, value.size())
+              .ToLocalChecked());
+    }
   }
 }
 
diff --git a/test/fixtures/dotenv/valid.env b/test/fixtures/dotenv/valid.env
index 56632b36ba82ff..c1c12b112b965b 100644
--- a/test/fixtures/dotenv/valid.env
+++ b/test/fixtures/dotenv/valid.env
@@ -31,5 +31,5 @@ RETAIN_INNER_QUOTES={"foo": "bar"}
 RETAIN_INNER_QUOTES_AS_STRING='{"foo": "bar"}'
 RETAIN_INNER_QUOTES_AS_BACKTICKS=`{"foo": "bar's"}`
 TRIM_SPACE_FROM_UNQUOTED=    some spaced out string
-USERNAME=therealnerdybeast@example.tld
+EMAIL=therealnerdybeast@example.tld
     SPACED_KEY = parsed
diff --git a/test/parallel/test-dotenv-edge-cases.js b/test/parallel/test-dotenv-edge-cases.js
index 1d256799bfbb13..ed7500953e0324 100644
--- a/test/parallel/test-dotenv-edge-cases.js
+++ b/test/parallel/test-dotenv-edge-cases.js
@@ -35,4 +35,17 @@ describe('.env supports edge cases', () => {
     assert.strictEqual(child.code, 0);
   });
 
+  it('should not override existing environment variables but introduce new vars', async () => {
+    const code = `
+      require('assert').strictEqual(process.env.BASIC, 'existing');
+      require('assert').strictEqual(process.env.AFTER_LINE, 'after_line');
+    `.trim();
+    const child = await common.spawnPromisified(
+      process.execPath,
+      [ `--env-file=${validEnvFilePath}`, '--eval', code ],
+      { cwd: __dirname, env: { ...process.env, BASIC: 'existing' } },
+    );
+    assert.strictEqual(child.stderr, '');
+    assert.strictEqual(child.code, 0);
+  });
 });
diff --git a/test/parallel/test-dotenv-node-options.js b/test/parallel/test-dotenv-node-options.js
index 7dbb1a423707db..d35d1eeaeb33db 100644
--- a/test/parallel/test-dotenv-node-options.js
+++ b/test/parallel/test-dotenv-node-options.js
@@ -48,10 +48,15 @@ describe('.env supports NODE_OPTIONS', () => {
     const code = `
       require('assert')(new Date().toString().includes('GMT-1000'))
     `.trim();
+    // Some CI environments set TZ. Since an env file doesn't override existing
+    // environment variables, we need to delete it and then pass the env object
+    // as the environment to spawnPromisified.
+    const env = { ...process.env };
+    delete env.TZ;
     const child = await common.spawnPromisified(
       process.execPath,
       [ `--env-file=${relativePath}`, '--eval', code ],
-      { cwd: __dirname },
+      { cwd: __dirname, env },
     );
     assert.strictEqual(child.stderr, '');
     assert.strictEqual(child.code, 0);
diff --git a/test/parallel/test-dotenv.js b/test/parallel/test-dotenv.js
index eb4d4178b1a4f4..9c374c8735910d 100644
--- a/test/parallel/test-dotenv.js
+++ b/test/parallel/test-dotenv.js
@@ -65,6 +65,6 @@ assert.strictEqual(process.env.RETAIN_INNER_QUOTES_AS_BACKTICKS, '{"foo": "bar\'
 // Retains spaces in string
 assert.strictEqual(process.env.TRIM_SPACE_FROM_UNQUOTED, 'some spaced out string');
 // Parses email addresses completely
-assert.strictEqual(process.env.USERNAME, 'therealnerdybeast@example.tld');
+assert.strictEqual(process.env.EMAIL, 'therealnerdybeast@example.tld');
 // Parses keys and values surrounded by spaces
 assert.strictEqual(process.env.SPACED_KEY, 'parsed');