diff --git a/lib/watch/index.js b/lib/watch/index.js
index 81de0b2f..9a9e8bb9 100644
--- a/lib/watch/index.js
+++ b/lib/watch/index.js
@@ -3,6 +3,7 @@
 const path = require('path')
 const cp = require('child_process')
 const chalk = require('chalk')
+const { readFileSync, existsSync } = require('fs')
 const { arrayToRegExp, logWatchVerbose } = require('./utils')
 const { GRACEFUL_SHUT } = require('./constants.js')
 
@@ -36,10 +37,34 @@ const watch = function (args, ignoreWatch, verboseWatch) {
   })
 
   let readyEmitted = false
+  const envVarsToProtect = [];
 
   const run = (event) => {
     const childEvent = { childEvent: event }
-    const env = Object.assign({}, process.env, childEvent, require('dotenv').config().parsed)
+
+    // in order to not override existing env vars, we have to load and parse
+    // dotenv without modifying process.env
+    let dotenvParsed = {};
+    const dotenvPath = path.resolve(process.cwd(), '.env');
+    if (existsSync(dotenvPath)) {
+      dotenvParsed = require('dotenv').parse(readFileSync(dotenvPath, { encoding: 'utf-8' }));
+      if (event == 'start') {
+        // analyse which env-vars should not be overwritten
+        Object.keys(dotenvParsed).forEach(function (key) {
+          if (Object.prototype.hasOwnProperty.call(process.env, key)) {
+            // if a env var already exists with a different value
+            // than in parsed dotenv, it has to be protected
+            if (process.env[key] != dotenvParsed[key]) {
+              envVarsToProtect.push(key);
+            }
+          }
+        });
+      }
+      for (const envVarToProtect of envVarsToProtect) {
+        delete dotenvParsed[envVarToProtect];
+      }
+    }
+    const env = Object.assign({}, process.env, dotenvParsed, childEvent);
 
     const _child = cp.fork(forkPath, args, {
       env,
diff --git a/test/start.test.js b/test/start.test.js
index 59299776..c8489f41 100644
--- a/test/start.test.js
+++ b/test/start.test.js
@@ -677,6 +677,38 @@ test('should read env variables from .env file', async (t) => {
   await fastify.close()
 })
 
+test('should read env variables from .env file and not override existing env vars in watch mode', async (t) => {
+  process.env.GREETING = 'planet'
+  const testdir = t.testdir({
+    '.env': 'GREETING=world',
+    'plugin.js': await readFile(path.join(__dirname, '../examples/plugin-with-env.js'))
+  })
+
+  const cwd = process.cwd()
+
+  process.chdir(testdir)
+
+  const port = getPort()
+  const argv = ['-p', port, '-w', path.join(testdir, 'plugin.js')]
+  const fastifyEmitter = await requireUncached('../start').start(argv)
+
+  t.teardown(() => {
+    process.chdir(cwd)
+  })
+
+  await once(fastifyEmitter, 'ready')
+
+  const r1 = await sget({
+    method: 'GET',
+    url: `http://localhost:${port}`
+  })
+
+  t.equal(r1.response.statusCode, 200)
+  t.same(JSON.parse(r1.body), { hello: 'planet' })
+
+  await fastifyEmitter.stop()
+})
+
 test('crash on unhandled rejection', t => {
   t.plan(1)