diff --git a/packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts b/packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts
index dd2ea23014aa..3abd95d55278 100644
--- a/packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts
+++ b/packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts
@@ -526,6 +526,13 @@ export function getCommonConfig(wco: WebpackConfigOptions): Configuration {
           test: /[\/\\]@angular[\/\\]core[\/\\].+\.js$/,
           parser: { system: true },
         },
+        {
+          // Mark files inside `rxjs/add` as containing side effects.
+          // If this is fixed upstream and the fixed version becomes the minimum
+          // supported version, this can be removed.
+          test: /[\/\\]rxjs[\/\\]add[\/\\].+\.js$/,
+          sideEffects: true,
+        },
         {
           test: /\.m?js$/,
           exclude: [/[\/\\](?:core-js|\@babel|tslib)[\/\\]/, /(ngfactory|ngstyle)\.js$/],