diff --git a/packages/block-editor/src/store/selectors.js b/packages/block-editor/src/store/selectors.js
index 787225f18a0a57..b5afb2891ef1cf 100644
--- a/packages/block-editor/src/store/selectors.js
+++ b/packages/block-editor/src/store/selectors.js
@@ -1628,18 +1628,8 @@ const isBlockVisibleInTheInserter = (
 	checkedBlocks.add( blockName );
 
 	// If parent blocks are not visible, child blocks should be hidden too.
-	//
-	// In some scenarios, blockType.parent may be a string.
-	// A better approach would be sanitize parent in all the places that can be modified:
-	// block registration, processBlockType, filters, etc.
-	// In the meantime, this is a hotfix to prevent the editor from crashing.
-	const parent =
-		typeof blockType.parent === 'string' ||
-		blockType.parent instanceof String
-			? [ blockType.parent ]
-			: blockType.parent;
-	if ( Array.isArray( parent ) ) {
-		return parent.some(
+	if ( Array.isArray( blockType.parent ) ) {
+		return blockType.parent.some(
 			( name ) =>
 				( blockName !== name &&
 					isBlockVisibleInTheInserter(
diff --git a/packages/blocks/CHANGELOG.md b/packages/blocks/CHANGELOG.md
index 73a700428674b2..eebbe7148e8055 100644
--- a/packages/blocks/CHANGELOG.md
+++ b/packages/blocks/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Breaking changes
+
+- Normalize `blockType.parent` to be an array. While string values were never supported, they appeared to work with some unintended side-effects that have been fixed by [#66250](https://github.com/WordPress/gutenberg/pull/66250). For that reason, we've added some code that automatically migrates strings to arrays — though it still raises a warning.
+
 ## 13.10.0 (2024-10-16)
 
 ## 13.9.0 (2024-10-03)
diff --git a/packages/blocks/src/api/registration.js b/packages/blocks/src/api/registration.js
index ef1b6bd20a4cdd..2f4bab2b5f2589 100644
--- a/packages/blocks/src/api/registration.js
+++ b/packages/blocks/src/api/registration.js
@@ -232,15 +232,6 @@ export function registerBlockType( blockNameOrMetadata, settings ) {
 		return;
 	}
 
-	if ( 1 === settings?.parent?.length && name === settings.parent[ 0 ] ) {
-		warning(
-			'Block "' +
-				name +
-				'" cannot be a parent of itself. Please remove the block name from the parent list.'
-		);
-		return;
-	}
-
 	if ( ! /^[a-z][a-z0-9-]*\/[a-z][a-z0-9-]*$/.test( name ) ) {
 		warning(
 			'Block names must contain a namespace prefix, include only lowercase alphanumeric characters or dashes, and start with a letter. Example: my-plugin/my-custom-block'
diff --git a/packages/blocks/src/api/test/registration.js b/packages/blocks/src/api/test/registration.js
index 656e9471a541b8..5941415e61fe55 100644
--- a/packages/blocks/src/api/test/registration.js
+++ b/packages/blocks/src/api/test/registration.js
@@ -742,6 +742,39 @@ describe( 'blocks', () => {
 			} );
 		} );
 
+		it( 'should transform parent string to array', () => {
+			const blockType = {
+				save: noop,
+				category: 'text',
+				title: 'block title',
+				parent: 'core/paragraph',
+			};
+			const block = registerBlockType(
+				'core/test-block-parent-string',
+				blockType
+			);
+			expect( console ).toHaveWarnedWith(
+				'Parent must be undefined or an array of strings (block types), but it is a string.'
+			);
+			expect( block ).toEqual( {
+				name: 'core/test-block-parent-string',
+				save: noop,
+				category: 'text',
+				title: 'block title',
+				icon: { src: BLOCK_ICON_DEFAULT },
+				attributes: {},
+				providesContext: {},
+				usesContext: [],
+				keywords: [],
+				selectors: {},
+				supports: {},
+				styles: [],
+				variations: [],
+				blockHooks: {},
+				parent: [ 'core/paragraph' ],
+			} );
+		} );
+
 		describe( 'applyFilters', () => {
 			afterEach( () => {
 				removeAllFilters( 'blocks.registerBlockType' );
diff --git a/packages/blocks/src/store/process-block-type.js b/packages/blocks/src/store/process-block-type.js
index ae8f92f84cfabb..bc7b1a0e10e774 100644
--- a/packages/blocks/src/store/process-block-type.js
+++ b/packages/blocks/src/store/process-block-type.js
@@ -197,5 +197,41 @@ export const processBlockType =
 			return;
 		}
 
+		if (
+			typeof settings?.parent === 'string' ||
+			settings?.parent instanceof String
+		) {
+			settings.parent = [ settings.parent ];
+			warning(
+				'Parent must be undefined or an array of strings (block types), but it is a string.'
+			);
+			// Intentionally continue:
+			//
+			// While string values were never supported, they appeared to work with some unintended side-effects
+			// that have been fixed by [#66250](https://github.com/WordPress/gutenberg/pull/66250).
+			//
+			// To be backwards-compatible, this code that automatically migrates strings to arrays.
+		}
+
+		if (
+			! Array.isArray( settings?.parent ) &&
+			settings?.parent !== undefined
+		) {
+			warning(
+				'Parent must be undefined or an array of block types, but it is ',
+				settings.parent
+			);
+			return;
+		}
+
+		if ( 1 === settings?.parent?.length && name === settings.parent[ 0 ] ) {
+			warning(
+				'Block "' +
+					name +
+					'" cannot be a parent of itself. Please remove the block name from the parent list.'
+			);
+			return;
+		}
+
 		return settings;
 	};