diff --git a/.changeset/great-dolphins-tie.md b/.changeset/great-dolphins-tie.md
new file mode 100644
index 00000000000..d6413844bfb
--- /dev/null
+++ b/.changeset/great-dolphins-tie.md
@@ -0,0 +1,5 @@
+---
+"@firebase/database": patch
+---
+
+The SDK can now infer a default database URL if none is provided in the config.
diff --git a/packages/database/src/core/RepoManager.ts b/packages/database/src/core/RepoManager.ts
index 5ec5e7bc753..a8d8032684e 100644
--- a/packages/database/src/core/RepoManager.ts
+++ b/packages/database/src/core/RepoManager.ts
@@ -18,7 +18,7 @@
 import { FirebaseApp } from '@firebase/app-types';
 import { safeGet, CONSTANTS } from '@firebase/util';
 import { Repo } from './Repo';
-import { fatal } from './util/util';
+import { fatal, log } from './util/util';
 import { parseRepoInfo } from './util/libs/parser';
 import { validateUrl } from './util/validation';
 import './Repo_transaction';
@@ -32,9 +32,6 @@ import {
   FirebaseAuthTokenProvider
 } from './AuthTokenProvider';
 
-/** @const {string} */
-const DATABASE_URL_OPTION = 'databaseURL';
-
 /**
  * This variable is also defined in the firebase node.js admin SDK. Before
  * modifying this definition, consult the definition in:
@@ -101,13 +98,17 @@ export class RepoManager {
     authProvider: Provider<FirebaseAuthInternalName>,
     url?: string
   ): Database {
-    let dbUrl: string | undefined = url || app.options[DATABASE_URL_OPTION];
+    let dbUrl: string | undefined = url || app.options.databaseURL;
     if (dbUrl === undefined) {
-      fatal(
-        "Can't determine Firebase Database URL.  Be sure to include " +
-          DATABASE_URL_OPTION +
-          ' option when calling firebase.initializeApp().'
-      );
+      if (!app.options.projectId) {
+        fatal(
+          "Can't determine Firebase Database URL. Be sure to include " +
+            ' a Project ID when calling firebase.initializeApp().'
+        );
+      }
+
+      log('Using default host for project ', app.options.projectId);
+      dbUrl = `${app.options.projectId}-default-rtdb.firebaseio.com`;
     }
 
     let parsedUrl = parseRepoInfo(dbUrl);
diff --git a/packages/database/test/database.test.ts b/packages/database/test/database.test.ts
index 1187032eb32..67dc3653462 100644
--- a/packages/database/test/database.test.ts
+++ b/packages/database/test/database.test.ts
@@ -105,6 +105,20 @@ describe('Database Tests', () => {
     expect(db.ref().toString()).to.equal('https://localhost/');
   });
 
+  it('Can infer database URL from project Id', async () => {
+    const app = firebase.initializeApp(
+      { projectId: 'abc123' },
+      'project-id-app'
+    );
+    const db = app.database();
+    expect(db).to.be.ok;
+    // The URL is assumed to be secure if no port is specified.
+    expect(db.ref().toString()).to.equal(
+      'https://abc123-default-rtdb.firebaseio.com/'
+    );
+    await app.delete();
+  });
+
   it('Can read ns query param', () => {
     const db = defaultApp.database('http://localhost:80/?ns=foo&unused=true');
     expect(db).to.be.ok;