diff --git a/packages/react-dom/package.json b/packages/react-dom/package.json
index c39432aa32046..baa9032ef4c62 100644
--- a/packages/react-dom/package.json
+++ b/packages/react-dom/package.json
@@ -38,6 +38,7 @@
     "server-rendering-stub.js",
     "test-utils.js",
     "unstable_testing.js",
+    "unstable_server-external-runtime.js",
     "cjs/",
     "umd/"
   ],
diff --git a/packages/react-dom/src/server/ReactDOMServerExternalRuntime.js b/packages/react-dom/src/server/ReactDOMServerExternalRuntime.js
new file mode 100644
index 0000000000000..dccb1bb3debe9
--- /dev/null
+++ b/packages/react-dom/src/server/ReactDOMServerExternalRuntime.js
@@ -0,0 +1,18 @@
+// TODO: Add Flow types
+import {
+  clientRenderBoundary,
+  completeBoundaryWithStyles,
+  completeBoundary,
+  completeSegment,
+} from 'react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSet';
+
+// Intentionally does nothing. Implementation will be added in future PR.
+// eslint-disable-next-line no-unused-vars
+const observer = new MutationObserver(mutations => {
+  // These are only called so I can check what the module output looks like. The
+  // code is unreachable.
+  clientRenderBoundary();
+  completeBoundaryWithStyles();
+  completeBoundary();
+  completeSegment();
+});
diff --git a/scripts/rollup/build.js b/scripts/rollup/build.js
index 52c0b7dc9fe29..b4aca4abde256 100644
--- a/scripts/rollup/build.js
+++ b/scripts/rollup/build.js
@@ -60,6 +60,7 @@ const {
   RN_FB_DEV,
   RN_FB_PROD,
   RN_FB_PROFILING,
+  BROWSER_SCRIPT,
 } = Bundles.bundleTypes;
 
 const {getFilename} = Bundles;
@@ -224,6 +225,8 @@ function getFormat(bundleType) {
       return `cjs`;
     case NODE_ESM:
       return `es`;
+    case BROWSER_SCRIPT:
+      return `iife`;
   }
 }
 
@@ -247,6 +250,7 @@ function isProductionBundleType(bundleType) {
     case RN_OSS_PROFILING:
     case RN_FB_PROD:
     case RN_FB_PROFILING:
+    case BROWSER_SCRIPT:
       return true;
     default:
       throw new Error(`Unknown type: ${bundleType}`);
@@ -267,6 +271,7 @@ function isProfilingBundleType(bundleType) {
     case RN_OSS_PROD:
     case UMD_DEV:
     case UMD_PROD:
+    case BROWSER_SCRIPT:
       return false;
     case FB_WWW_PROFILING:
     case NODE_PROFILING:
@@ -582,6 +587,7 @@ async function createBundle(bundle, bundleType) {
     },
   };
   const mainOutputPath = Packaging.getBundleOutputPath(
+    bundle,
     bundleType,
     filename,
     packageName
@@ -724,7 +730,8 @@ async function buildEverything() {
       [bundle, RN_OSS_PROFILING],
       [bundle, RN_FB_DEV],
       [bundle, RN_FB_PROD],
-      [bundle, RN_FB_PROFILING]
+      [bundle, RN_FB_PROFILING],
+      [bundle, BROWSER_SCRIPT]
     );
   }
 
diff --git a/scripts/rollup/bundles.js b/scripts/rollup/bundles.js
index 1f7297a8f2174..81072166a0269 100644
--- a/scripts/rollup/bundles.js
+++ b/scripts/rollup/bundles.js
@@ -25,6 +25,7 @@ const bundleTypes = {
   RN_FB_DEV: 'RN_FB_DEV',
   RN_FB_PROD: 'RN_FB_PROD',
   RN_FB_PROFILING: 'RN_FB_PROFILING',
+  BROWSER_SCRIPT: 'BROWSER_SCRIPT',
 };
 
 const {
@@ -45,6 +46,7 @@ const {
   RN_FB_DEV,
   RN_FB_PROD,
   RN_FB_PROFILING,
+  BROWSER_SCRIPT,
 } = bundleTypes;
 
 const moduleTypes = {
@@ -351,6 +353,18 @@ const bundles = [
     externals: ['react', 'util', 'stream', 'react-dom'],
   },
 
+  /******* React DOM Fizz Server External Runtime *******/
+  {
+    bundleTypes: [BROWSER_SCRIPT],
+    moduleType: RENDERER,
+    entry: 'react-dom/src/server/ReactDOMServerExternalRuntime.js',
+    outputPath: 'unstable_server-external-runtime.js',
+    global: 'ReactDOMServerExternalRuntime',
+    minifyWithProdErrorCodes: false,
+    wrapWithModuleBoundaries: false,
+    externals: [],
+  },
+
   /******* React DOM Server Render Stub *******/
   {
     bundleTypes: [NODE_DEV, NODE_PROD, UMD_DEV, UMD_PROD],
@@ -1030,6 +1044,8 @@ function getOriginalFilename(bundle, bundleType) {
     case RN_FB_PROFILING:
     case RN_OSS_PROFILING:
       return `${globalName}-profiling.js`;
+    case BROWSER_SCRIPT:
+      return `${name}.js`;
   }
 }
 
diff --git a/scripts/rollup/packaging.js b/scripts/rollup/packaging.js
index bd66d0b634730..b09dd0ce509d8 100644
--- a/scripts/rollup/packaging.js
+++ b/scripts/rollup/packaging.js
@@ -33,6 +33,7 @@ const {
   RN_FB_DEV,
   RN_FB_PROD,
   RN_FB_PROFILING,
+  BROWSER_SCRIPT,
 } = Bundles.bundleTypes;
 
 function getPackageName(name) {
@@ -42,7 +43,7 @@ function getPackageName(name) {
   return name;
 }
 
-function getBundleOutputPath(bundleType, filename, packageName) {
+function getBundleOutputPath(bundle, bundleType, filename, packageName) {
   switch (bundleType) {
     case NODE_ES2015:
       return `build/node_modules/${packageName}/cjs/${filename}`;
@@ -88,6 +89,24 @@ function getBundleOutputPath(bundleType, filename, packageName) {
         default:
           throw new Error('Unknown RN package.');
       }
+    case BROWSER_SCRIPT: {
+      // Bundles that are served as browser scripts need to be able to be sent
+      // straight to the browser with any additional bundling. We shouldn't use
+      // a module to re-export. Depending on how they are served, they also may
+      // not go through package.json module resolution, so we shouldn't rely on
+      // that either. We should consider the output path as part of the public
+      // contract, and explicitly specify its location within the package's
+      // directory structure.
+      const outputPath = bundle.outputPath;
+      if (!outputPath) {
+        throw new Error(
+          'Bundles with type BROWSER_SCRIPT must specific an explicit ' +
+            'output path.'
+        );
+      }
+      console.log(packageName);
+      return `build/node_modules/${packageName}/${outputPath}`;
+    }
     default:
       throw new Error('Unknown bundle type.');
   }
diff --git a/scripts/rollup/wrappers.js b/scripts/rollup/wrappers.js
index 44d83e4f58ee0..e73fb01546312 100644
--- a/scripts/rollup/wrappers.js
+++ b/scripts/rollup/wrappers.js
@@ -22,6 +22,7 @@ const {
   RN_FB_DEV,
   RN_FB_PROD,
   RN_FB_PROFILING,
+  BROWSER_SCRIPT,
 } = bundleTypes;
 
 const {RECONCILER} = moduleTypes;
@@ -384,6 +385,12 @@ function wrapBundle(
     }
   }
 
+  if (bundleType === BROWSER_SCRIPT) {
+    // Bundles of type BROWSER_SCRIPT get sent straight to the browser without
+    // additional processing. So we should exclude any extra wrapper comments.
+    return source;
+  }
+
   if (moduleType === RECONCILER) {
     // Standalone reconciler is only used by third-party renderers.
     // It is handled separately.
@@ -395,6 +402,7 @@ function wrapBundle(
     }
     return wrapper(source, globalName, filename, moduleType);
   }
+
   // All the other packages.
   const wrapper = wrappers[bundleType];
   if (typeof wrapper !== 'function') {
diff --git a/scripts/shared/inlinedHostConfigs.js b/scripts/shared/inlinedHostConfigs.js
index 1113bad1247d0..027c71aa173b3 100644
--- a/scripts/shared/inlinedHostConfigs.js
+++ b/scripts/shared/inlinedHostConfigs.js
@@ -15,6 +15,7 @@ module.exports = [
       'react-dom/src/server/ReactDOMFizzServerNode.js',
       'react-dom/static.node',
       'react-dom/server-rendering-stub',
+      'react-dom/src/server/ReactDOMServerExternalRuntime.js',
       'react-server-dom-webpack/writer.node.server',
       'react-server-dom-webpack',
     ],
@@ -51,6 +52,7 @@ module.exports = [
       'react-dom/src/server/ReactDOMFizzServerBrowser.js',
       'react-dom/static.browser',
       'react-dom/server-rendering-stub',
+      'react-dom/src/server/ReactDOMServerExternalRuntime.js',
       'react-server-dom-webpack/writer.browser.server',
       'react-server-dom-webpack',
     ],