diff --git a/src/api/project/dto/update-project.dto.spec.ts b/src/api/project/dto/update-project.dto.spec.ts
new file mode 100644
index 0000000..9132e10
--- /dev/null
+++ b/src/api/project/dto/update-project.dto.spec.ts
@@ -0,0 +1,26 @@
+import { plainToInstance } from 'class-transformer';
+import { validate } from 'class-validator';
+import { UpdateProjectDto } from './update-project.dto';
+
+describe('UpdateProjectDto', () => {
+  it.each([null, ''])(
+    'preserves %p billingAccountId as an explicit clear request',
+    async (billingAccountId) => {
+      const dto = plainToInstance(UpdateProjectDto, {
+        billingAccountId,
+      });
+
+      expect(dto.billingAccountId).toBeNull();
+      await expect(validate(dto)).resolves.toHaveLength(0);
+    },
+  );
+
+  it('parses numeric billingAccountId updates', async () => {
+    const dto = plainToInstance(UpdateProjectDto, {
+      billingAccountId: '80001063',
+    });
+
+    expect(dto.billingAccountId).toBe(80001063);
+    await expect(validate(dto)).resolves.toHaveLength(0);
+  });
+});
diff --git a/src/api/project/dto/update-project.dto.ts b/src/api/project/dto/update-project.dto.ts
index 80f6b13..b4aea56 100644
--- a/src/api/project/dto/update-project.dto.ts
+++ b/src/api/project/dto/update-project.dto.ts
@@ -1,8 +1,33 @@
-import { ApiHideProperty } from '@nestjs/swagger';
+import { ApiHideProperty, ApiPropertyOptional } from '@nestjs/swagger';
 import { Transform } from 'class-transformer';
-import { PartialType } from '@nestjs/mapped-types';
+import { IsNumber, IsOptional } from 'class-validator';
+import { OmitType, PartialType } from '@nestjs/mapped-types';
 import { CreateProjectDto } from './create-project.dto';
 
+/**
+ * Parses optional project billing-account update input.
+ *
+ * @param value Raw `billingAccountId` value from request payload.
+ * @returns Parsed integer, `null` when clearing, or `undefined` when omitted.
+ */
+function parseOptionalNullableInteger(value: unknown): number | null | undefined {
+  if (typeof value === 'undefined') {
+    return undefined;
+  }
+
+  if (value === null || value === '') {
+    return null;
+  }
+
+  const parsed = Number(value);
+
+  if (Number.isNaN(parsed)) {
+    return undefined;
+  }
+
+  return Math.trunc(parsed);
+}
+
 /**
  * Resolves whether the patch payload explicitly requests clearing
  * `billingAccountId`.
@@ -17,9 +42,21 @@ function parseClearBillingAccountFlag(value: unknown): boolean {
 /**
  * Request DTO for `PATCH /projects/:projectId`.
  *
- * Reuses `CreateProjectDto` and makes all fields optional via `PartialType`.
+ * Reuses `CreateProjectDto`, makes all fields optional via `PartialType`, and
+ * allows `billingAccountId` to be explicitly cleared with `null` or `''`.
  */
-export class UpdateProjectDto extends PartialType(CreateProjectDto) {
+export class UpdateProjectDto extends PartialType(
+  OmitType(CreateProjectDto, ['billingAccountId'] as const),
+) {
+  @ApiPropertyOptional({
+    description: 'Project billing account id. Send null or empty string to clear.',
+    nullable: true,
+  })
+  @IsOptional()
+  @Transform(({ value }) => parseOptionalNullableInteger(value))
+  @IsNumber()
+  billingAccountId?: number | null;
+
   @ApiHideProperty()
   @Transform(({ obj }) => parseClearBillingAccountFlag(obj?.billingAccountId))
   clearBillingAccountId?: boolean;
diff --git a/src/api/project/project.service.spec.ts b/src/api/project/project.service.spec.ts
index fc4e9a4..9df8331 100644
--- a/src/api/project/project.service.spec.ts
+++ b/src/api/project/project.service.spec.ts
@@ -1646,8 +1646,8 @@ describe('ProjectService', () => {
     await service.updateProject(
       '1001',
       {
-        clearBillingAccountId: true,
-      } as any,
+        billingAccountId: null,
+      },
       {
         userId: '100',
         isMachine: false,
diff --git a/src/api/project/project.service.ts b/src/api/project/project.service.ts
index de3d963..9da39d1 100644
--- a/src/api/project/project.service.ts
+++ b/src/api/project/project.service.ts
@@ -666,8 +666,10 @@ export class ProjectService {
       throw new ForbiddenException('Insufficient permissions');
     }
 
+    const shouldClearBillingAccountId =
+      dto.clearBillingAccountId === true || dto.billingAccountId === null;
     const requestedBillingAccountId =
-      dto.clearBillingAccountId === true
+      shouldClearBillingAccountId
         ? null
         : typeof dto.billingAccountId === 'number'
           ? String(dto.billingAccountId)
@@ -726,7 +728,7 @@ export class ProjectService {
           cancelReason:
             typeof dto.cancelReason === 'string' ? dto.cancelReason : undefined,
           billingAccountId:
-            dto.clearBillingAccountId === true
+            shouldClearBillingAccountId
               ? null
               : typeof dto.billingAccountId === 'number'
                 ? BigInt(dto.billingAccountId)