diff --git a/src/edit.js b/src/edit.js
index 538f80c..6f81520 100644
--- a/src/edit.js
+++ b/src/edit.js
@@ -59,7 +59,11 @@ export default function Edit( { attributes, setAttributes, clientId } ) {
 		businessUnitId,
 	} = attributes;
 
-	const [ isGlobalChanged, setIsGlobalChanged ] = useState( false );
+	// Tracks which of the global settings the user has edited, so that saving
+	// promotes only those values and leaves the other globals untouched.
+	const [ changedGlobals, setChangedGlobals ] = useState( () => new Set() );
+	const markGlobalChanged = ( key ) =>
+		setChangedGlobals( ( changed ) => new Set( changed ).add( key ) );
 
 	const isActiveContext = useSelect(
 		( select ) => {
@@ -116,12 +120,25 @@ export default function Edit( { attributes, setAttributes, clientId } ) {
 
 	const { insertBlock } = useDispatch( 'core/block-editor' );
 	const { saveSite } = useDispatch( 'core' );
-	const updateDefaults = ( newPortalId, newRegion, newBusinessUnitId ) =>
-		saveSite( {
-			hubspot_embed_portal_id: newPortalId,
-			hubspot_embed_region: newRegion,
-			hubspot_embed_business_unit_id: newBusinessUnitId || 0,
-		} );
+
+	// Build the settings payload from the fields the user actually filled in.
+	// An empty field means "inherit the global default shown as the
+	// placeholder", so it must not be sent. Sending every field unconditionally
+	// broke both ways: a blank ID goes over the wire as null, which the settings
+	// endpoint rejects with rest_invalid_stored_value once the option holds a
+	// value, aborting the entire save; and coercing a blank to 0 instead
+	// overwrote a stored global with an empty value.
+	const globalUpdates = {};
+	if ( changedGlobals.has( 'portalId' ) && portalId ) {
+		globalUpdates.hubspot_embed_portal_id = portalId;
+	}
+	if ( changedGlobals.has( 'businessUnitId' ) && businessUnitId ) {
+		globalUpdates.hubspot_embed_business_unit_id = businessUnitId;
+	}
+	if ( changedGlobals.has( 'region' ) && region ) {
+		globalUpdates.hubspot_embed_region = region;
+	}
+	const hasGlobalUpdates = Object.keys( globalUpdates ).length > 0;
 
 	return (
 		<div { ...innerBlocksProps }>
@@ -139,7 +156,7 @@ export default function Edit( { attributes, setAttributes, clientId } ) {
 							setAttributes( {
 								portalId: isNaN( parsed ) ? null : parsed,
 							} );
-							setIsGlobalChanged( true );
+							markGlobalChanged( 'portalId' );
 						} }
 						required={ ! defaultPortalId }
 					/>
@@ -152,7 +169,7 @@ export default function Edit( { attributes, setAttributes, clientId } ) {
 							setAttributes( {
 								businessUnitId: isNaN( parsed ) ? null : parsed,
 							} );
-							setIsGlobalChanged( true );
+							markGlobalChanged( 'businessUnitId' );
 						} }
 					/>
 					<SelectControl
@@ -174,23 +191,27 @@ export default function Edit( { attributes, setAttributes, clientId } ) {
 						defaultValue={ defaultRegion }
 						onChange={ ( newRegion ) => {
 							setAttributes( { region: newRegion } );
-							setIsGlobalChanged( true );
+							markGlobalChanged( 'region' );
 						} }
 					/>
-					{ canSetPortalId && isGlobalChanged && (
+					{ canSetPortalId && hasGlobalUpdates && (
 						<Button
 							variant="secondary"
 							onClick={ () => {
-								setAttributes( {
-									portalId: null,
-									businessUnitId: null,
-								} );
-								updateDefaults(
-									portalId,
-									region,
-									businessUnitId
-								);
-								setIsGlobalChanged( false );
+								// Clear only the block-level overrides that
+								// are being promoted to global defaults.
+								const clearedAttributes = {};
+								if ( globalUpdates.hubspot_embed_portal_id ) {
+									clearedAttributes.portalId = null;
+								}
+								if (
+									globalUpdates.hubspot_embed_business_unit_id
+								) {
+									clearedAttributes.businessUnitId = null;
+								}
+								setAttributes( clearedAttributes );
+								saveSite( globalUpdates );
+								setChangedGlobals( new Set() );
 							} }
 						>
 							{ __(
@@ -275,7 +296,7 @@ export default function Edit( { attributes, setAttributes, clientId } ) {
 				</svg>
 				{ __( 'Hubspot Form', 'hubspot-form-block' ) }
 			</h3>
-			{ ( ! defaultPortalId || ! formId ) && (
+			{ ( ! ( portalId || defaultPortalId ) || ! formId ) && (
 				<p>
 					{ __(
 						'Please enter a Portal ID and a Form ID in the sidebar block controls.',
@@ -283,7 +304,7 @@ export default function Edit( { attributes, setAttributes, clientId } ) {
 					) }
 				</p>
 			) }
-			{ defaultPortalId && formId && (
+			{ ( portalId || defaultPortalId ) && formId && (
 				<p>
 					{ __(
 						'Please preview your changes to see the form, it cannot be shown in the editor directly.',
diff --git a/tests/global-settings.spec.js b/tests/global-settings.spec.js
new file mode 100644
index 0000000..051d486
--- /dev/null
+++ b/tests/global-settings.spec.js
@@ -0,0 +1,223 @@
+/**
+ * WordPress dependencies
+ */
+const { test, expect } = require( '@wordpress/e2e-test-utils-playwright' );
+
+/**
+ * Internal dependencies
+ */
+const { editorCanvas } = require( './helpers' );
+
+const PORTAL_ID = 148262752;
+const FORM_ID = 'ec0707d2-b7f5-47c5-bfef-76eb7e8f837e';
+
+// 0 is the "unset" value for the ID options — the plugin treats it as empty and
+// the editor falls back to a blank placeholder. Writing null instead would fail
+// with rest_invalid_stored_value whenever the option already has a value.
+const EMPTY_GLOBALS = {
+	hubspot_embed_portal_id: 0,
+	hubspot_embed_business_unit_id: 0,
+	hubspot_embed_region: 'eu1',
+};
+
+/**
+ * Reads the plugin's global settings from the REST API.
+ *
+ * @param {Object} requestUtils The e2e request utils fixture.
+ * @return {Promise<Object>} The HubSpot embed settings.
+ */
+async function getGlobals( requestUtils ) {
+	const settings = await requestUtils.rest( { path: '/wp/v2/settings' } );
+	return {
+		portalId: settings.hubspot_embed_portal_id,
+		businessUnitId: settings.hubspot_embed_business_unit_id,
+		region: settings.hubspot_embed_region,
+	};
+}
+
+/**
+ * Writes the plugin's global settings via the REST API.
+ *
+ * @param {Object} requestUtils The e2e request utils fixture.
+ * @param {Object} data         Settings to write.
+ */
+async function setGlobals( requestUtils, data ) {
+	await requestUtils.rest( {
+		path: '/wp/v2/settings',
+		method: 'POST',
+		data,
+	} );
+}
+
+/**
+ * Inserts the block and reveals its "Global Settings" panel in the inspector.
+ *
+ * The panel's initialOpen depends on whether a global Portal ID is set, so it is
+ * expanded explicitly rather than assumed.
+ *
+ * @param {Object} admin      The e2e admin fixture.
+ * @param {Object} editor     The e2e editor fixture.
+ * @param {Object} page       The Playwright page.
+ * @param {Object} attributes Block attributes to insert with.
+ */
+async function insertBlockAndOpenGlobalSettings(
+	admin,
+	editor,
+	page,
+	attributes = {}
+) {
+	await admin.createNewPost();
+	await editor.setPreferences( 'core/edit-post', { welcomeGuide: false } );
+	await editor.insertBlock( { name: 'hubspot/form', attributes } );
+	await editor.openDocumentSettingsSidebar();
+
+	const blockTab = page.getByRole( 'tab', { name: 'Block' } );
+	if ( await blockTab.isVisible() ) {
+		await blockTab.click();
+	}
+
+	const panelToggle = page.getByRole( 'button', { name: 'Global Settings' } );
+	await expect( panelToggle ).toBeVisible();
+	if ( ( await panelToggle.getAttribute( 'aria-expanded' ) ) === 'false' ) {
+		await panelToggle.click();
+	}
+}
+
+test.describe( 'HubSpot Form Block global settings', () => {
+	test.beforeEach( async ( { requestUtils } ) => {
+		await setGlobals( requestUtils, EMPTY_GLOBALS );
+	} );
+
+	test.afterEach( async ( { requestUtils } ) => {
+		await setGlobals( requestUtils, EMPTY_GLOBALS );
+	} );
+
+	test( 'saving a Business Unit ID leaves an existing global Portal ID intact', async ( {
+		admin,
+		editor,
+		page,
+		requestUtils,
+	} ) => {
+		// A Portal ID saved earlier, as in the reported sequence.
+		await setGlobals( requestUtils, {
+			hubspot_embed_portal_id: PORTAL_ID,
+		} );
+
+		await insertBlockAndOpenGlobalSettings( admin, editor, page );
+
+		// The saved global is offered as the placeholder, so the field is left
+		// blank — only the Business Unit ID is entered.
+		const portalInput = page.getByLabel( 'Portal ID' );
+		await expect( portalInput ).toHaveValue( '' );
+		await expect( portalInput ).toHaveAttribute(
+			'placeholder',
+			String( PORTAL_ID )
+		);
+
+		await page.getByLabel( 'Business Unit ID' ).fill( '99' );
+		await page
+			.getByRole( 'button', { name: 'Set as global defaults' } )
+			.click();
+
+		await expect
+			.poll(
+				async () => ( await getGlobals( requestUtils ) ).businessUnitId
+			)
+			.toBe( 99 );
+
+		// The blank Portal ID field must not have disturbed the saved global.
+		const globals = await getGlobals( requestUtils );
+		expect( globals.portalId ).toBe( PORTAL_ID );
+		expect( globals.region ).toBe( 'eu1' );
+	} );
+
+	test( 'saving a Portal ID on its own stores just that setting', async ( {
+		admin,
+		editor,
+		page,
+		requestUtils,
+	} ) => {
+		await insertBlockAndOpenGlobalSettings( admin, editor, page );
+
+		await page.getByLabel( 'Portal ID' ).fill( String( PORTAL_ID ) );
+		await page
+			.getByRole( 'button', { name: 'Set as global defaults' } )
+			.click();
+
+		await expect
+			.poll( async () => ( await getGlobals( requestUtils ) ).portalId )
+			.toBe( PORTAL_ID );
+
+		// The untouched Business Unit ID was never sent, so it stays unset.
+		const globals = await getGlobals( requestUtils );
+		expect( globals.businessUnitId ).toBe( 0 );
+	} );
+
+	test( 'saving a global setting does not reset a non-default region', async ( {
+		admin,
+		editor,
+		page,
+		requestUtils,
+	} ) => {
+		await setGlobals( requestUtils, { hubspot_embed_region: 'na1' } );
+
+		await insertBlockAndOpenGlobalSettings( admin, editor, page );
+
+		await page.getByLabel( 'Portal ID' ).fill( String( PORTAL_ID ) );
+		await page
+			.getByRole( 'button', { name: 'Set as global defaults' } )
+			.click();
+
+		await expect
+			.poll( async () => ( await getGlobals( requestUtils ) ).portalId )
+			.toBe( PORTAL_ID );
+
+		// The region was never touched in the sidebar, so it must be left alone
+		// rather than reset to the block attribute default.
+		const globals = await getGlobals( requestUtils );
+		expect( globals.region ).toBe( 'na1' );
+	} );
+
+	test( 'the save button only appears once a global value is entered', async ( {
+		admin,
+		editor,
+		page,
+	} ) => {
+		await insertBlockAndOpenGlobalSettings( admin, editor, page );
+
+		const portalInput = page.getByLabel( 'Portal ID' );
+		const saveButton = page.getByRole( 'button', {
+			name: 'Set as global defaults',
+		} );
+		await expect( saveButton ).toBeHidden();
+
+		await portalInput.fill( String( PORTAL_ID ) );
+		await expect( saveButton ).toBeVisible();
+
+		// Clearing the field again leaves nothing to promote, so there is no way
+		// to submit a blank value over a saved global.
+		await portalInput.fill( '' );
+		await expect( saveButton ).toBeHidden();
+	} );
+
+	test( 'a block-level Portal ID satisfies the editor notice without a global', async ( {
+		admin,
+		editor,
+		page,
+	} ) => {
+		await insertBlockAndOpenGlobalSettings( admin, editor, page, {
+			portalId: PORTAL_ID,
+			formId: FORM_ID,
+		} );
+
+		// No global Portal ID is set, but the block supplies its own, so the
+		// setup prompt must not be shown.
+		const block = editorCanvas( page ).getByRole( 'document', {
+			name: 'Block: Hubspot Form',
+		} );
+		await expect(
+			block.getByText( /Please preview your changes/i )
+		).toBeVisible();
+		await expect( block.getByText( /Portal ID.*Form ID/i ) ).toBeHidden();
+	} );
+} );