diff --git a/.changeset/rich-ears-bathe.md b/.changeset/rich-ears-bathe.md
new file mode 100644
index 0000000000..b86c884d39
--- /dev/null
+++ b/.changeset/rich-ears-bathe.md
@@ -0,0 +1,6 @@
+---
+'@directus/api': patch
+---
+
+Fixed deep `groupBy` queries for O2M relations, where results were not correctly grouped under their respective parent
+items
diff --git a/api/src/database/get-ast-from-query/lib/parse-fields.ts b/api/src/database/get-ast-from-query/lib/parse-fields.ts
index 4d8be36e09..478b4d8ef5 100644
--- a/api/src/database/get-ast-from-query/lib/parse-fields.ts
+++ b/api/src/database/get-ast-from-query/lib/parse-fields.ts
@@ -4,7 +4,7 @@ import type { Knex } from 'knex';
 import { isEmpty } from 'lodash-es';
 import { fetchPermissions } from '../../../permissions/lib/fetch-permissions.js';
 import { fetchPolicies } from '../../../permissions/lib/fetch-policies.js';
-import type { FieldNode, FunctionFieldNode, NestedCollectionNode } from '../../../types/index.js';
+import type { FieldNode, FunctionFieldNode, NestedCollectionNode, O2MNode } from '../../../types/index.js';
 import { getRelationType } from '../../../utils/get-relation-type.js';
 import { getDeepQuery } from '../utils/get-deep-query.js';
 import { getRelatedCollection } from '../utils/get-related-collection.js';
@@ -252,8 +252,14 @@ export async function parseFields(
 				whenCase: [],
 			};
 
-			if (relationType === 'o2m' && !child!.query.sort) {
-				child!.query.sort = [relation.meta?.sort_field || context.schema.collections[relation.collection]!.primary];
+			if (isO2MNode(child) && !child.query.sort) {
+				child.query.sort = [relation.meta?.sort_field || context.schema.collections[relation.collection]!.primary];
+			}
+
+			if (isO2MNode(child) && child?.query.group && child.query.group[0] !== relation.field) {
+				// If a group by is used, the result needs to be grouped by the foreign key of the relation first, so results
+				// are correctly grouped under the foreign key when extracting the grouped results from the nested queries.
+				child.query.group.unshift(relation.field);
 			}
 		}
 
@@ -275,3 +281,7 @@ export async function parseFields(
 		return true;
 	});
 }
+
+export function isO2MNode(node: NestedCollectionNode | null): node is O2MNode {
+	return !!node && node.type === 'o2m';
+}