diff --git a/packages/next/README.md b/packages/next/README.md
index 6cab569627ed1..b80638fdaaf30 100644
--- a/packages/next/README.md
+++ b/packages/next/README.md
@@ -1551,17 +1551,18 @@ The `modules` option on `"preset-env"` should be kept to `false` otherwise webpa
 
 ### Exposing configuration to the server / client side
 
-Configuration values can be replaced by webpack at build time.
+Configuration values can be replaced by Webpack at build time.
 
 ```js
 // next.config.js
 module.exports = {
   env: {
-    customKey: "value"
+    customKey: "value",
+    _globalVar: "value"
   }
 }
 ```
-This will be set to `proccess.env.customKey`.
+This will be set to `proccess.env.customKey` and `globalVar`, respectively.
 
 #### When configuration must be dynamic
 
diff --git a/packages/next/build/webpack-config.js b/packages/next/build/webpack-config.js
index f801a5f683b97..20e5014355ae3 100644
--- a/packages/next/build/webpack-config.js
+++ b/packages/next/build/webpack-config.js
@@ -294,10 +294,13 @@ export default async function getBaseWebpackConfig (dir, {dev = false, isServer
       new webpack.DefinePlugin(Object.assign(
         {},
         config.env ? Object.keys(config.env)
-          .reduce((acc, key) => ({
-            ...acc,
-            ...{ [`process.env.${key}`]: JSON.stringify(config.env[key]) }
-          }), {}) : {},
+          .reduce((acc, key) => {
+            const selector = key.match(/^(_?)(.+)$/)
+            return {
+              ...acc,
+              ...{ [`${selector[1] ? '' : 'process.env.'}${selector[2]}`]: JSON.stringify(config.env[key]) }
+            }
+          }, {}) : {},
         {
           'process.crossOrigin': JSON.stringify(config.crossOrigin),
           'process.browser': JSON.stringify(!isServer)