diff --git a/__tests__/serializers/serializeResourceLinkage.test.ts b/__tests__/serializers/serializeResourceLinkage.test.ts
index e86ecec..efe6f47 100644
--- a/__tests__/serializers/serializeResourceLinkage.test.ts
+++ b/__tests__/serializers/serializeResourceLinkage.test.ts
@@ -18,6 +18,19 @@ describe('`serializeResourceLinkage`', () => {
     chance = new Chance();
   });
 
+  describe('when given a resource linkage', () => {
+    it('should return the resource linkage as is', () => {
+      const resourceLinkage = {
+        type: chance.string(),
+        id: chance.string(),
+      };
+
+      expect(serializeResourceLinkage(resourceLinkage)).toEqual(
+        resourceLinkage,
+      );
+    });
+  });
+
   describe('when given an array of class instances', () => {
     it('should throw an error if some element in the array is not an object', () => {
       const classInstances = [1];
diff --git a/__tests__/serializers/utils/isResourceIdentifierObject.test.ts b/__tests__/serializers/utils/isResourceIdentifierObject.test.ts
new file mode 100644
index 0000000..abb6743
--- /dev/null
+++ b/__tests__/serializers/utils/isResourceIdentifierObject.test.ts
@@ -0,0 +1,80 @@
+import { isResourceIdentifierObject } from '../../../src/serializers/utils/isResourceIdentifierObject';
+
+describe('`isResourceIdentifierObject`', () => {
+  it('it should return `false` if `resourceIdentifierObjectCandidate` is null', () => {
+    const resourceIdentifierObjectCandidate = null;
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      false,
+    );
+  });
+
+  it('it should return `false` if `resourceIdentifierObjectCandidate` is not an object', () => {
+    const resourceIdentifierObjectCandidate = 'not an object';
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      false,
+    );
+  });
+
+  it('it should return `false` if `resourceIdentifierObjectCandidate` is an array', () => {
+    const resourceIdentifierObjectCandidate: unknown[] = [];
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      false,
+    );
+  });
+
+  it('it should return `false` if `resourceIdentifierObjectCandidate` does not have a `type` property', () => {
+    const resourceIdentifierObjectCandidate = {
+      id: '1',
+    };
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      false,
+    );
+  });
+
+  it('it should return `false` if `resourceIdentifierObjectCandidate.type` is not a string', () => {
+    const resourceIdentifierObjectCandidate = {
+      type: 1,
+      id: '1',
+    };
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      false,
+    );
+  });
+
+  it('it should return `false` if `resourceIdentifierObjectCandidate` does not have an `id` property', () => {
+    const resourceIdentifierObjectCandidate = {
+      type: 'test',
+    };
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      false,
+    );
+  });
+
+  it('it should return `false` if `resourceIdentifierObjectCandidate.id` is not a string', () => {
+    const resourceIdentifierObjectCandidate = {
+      type: 'test',
+      id: 1,
+    };
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      false,
+    );
+  });
+
+  it('it should return `true` if `resourceIdentifierObjectCandidate` is a valid JSON:API Resource Identifier Object', () => {
+    const resourceIdentifierObjectCandidate = {
+      type: 'test',
+      id: '1',
+    };
+
+    expect(isResourceIdentifierObject(resourceIdentifierObjectCandidate)).toBe(
+      true,
+    );
+  });
+});
diff --git a/__tests__/serializers/utils/isResourceLinkage.test.ts b/__tests__/serializers/utils/isResourceLinkage.test.ts
new file mode 100644
index 0000000..2da21b7
--- /dev/null
+++ b/__tests__/serializers/utils/isResourceLinkage.test.ts
@@ -0,0 +1,33 @@
+import { isResourceLinkage } from '../../../src/serializers/utils/isResourceLinkage';
+
+import { isResourceIdentifierObject } from '../../../src/serializers/utils/isResourceIdentifierObject';
+
+jest.mock('../../../src/serializers/utils/isResourceIdentifierObject');
+
+const isResourceIdentifierObjectMocked = jest.mocked(
+  isResourceIdentifierObject,
+);
+
+describe('`isResourceLinkage`', () => {
+  it('should return true if `resourceLinkageCandidate` is `null`', () => {
+    const resourceLinkageCandidate = null;
+
+    expect(isResourceLinkage(resourceLinkageCandidate)).toBe(true);
+  });
+
+  it('should return true if `resourceLinkageCandidate` is an array of `JSONAPIResourceIdentifierObject`', () => {
+    isResourceIdentifierObjectMocked.mockReturnValue(true);
+
+    const resourceLinkageCandidate = ['foo', 'bar'];
+
+    expect(isResourceLinkage(resourceLinkageCandidate)).toBe(true);
+  });
+
+  it('should return true if `resourceLinkageCandidate` is a `JSONAPIResourceIdentifierObject`', () => {
+    isResourceIdentifierObjectMocked.mockReturnValue(true);
+
+    const resourceLinkageCandidate = 'foo';
+
+    expect(isResourceLinkage(resourceLinkageCandidate)).toBe(true);
+  });
+});
diff --git a/src/serializers/serializeResourceLinkage.ts b/src/serializers/serializeResourceLinkage.ts
index 4d6cd3b..9776233 100644
--- a/src/serializers/serializeResourceLinkage.ts
+++ b/src/serializers/serializeResourceLinkage.ts
@@ -3,12 +3,17 @@ import { resourceSymbol } from '../decorators/resource';
 import { collect } from './utils/collect';
 import { getMetadataBySymbol } from './utils/getMetadataBySymbol';
 import { isObject } from './utils/isObject';
+import { isResourceLinkage } from './utils/isResourceLinkage';
 
 import type { JSONAPIResourceLinkage } from '../types/resourceLinkage';
 
 export const serializeResourceLinkage = <I extends object>(
-  classInstance_s: I,
-): Exclude<JSONAPIResourceLinkage, null> => {
+  classInstance_s: I | JSONAPIResourceLinkage,
+): JSONAPIResourceLinkage => {
+  if (isResourceLinkage(classInstance_s)) {
+    return classInstance_s;
+  }
+
   if (Array.isArray(classInstance_s)) {
     if (!classInstance_s.every(isObject)) {
       throw new Error(
diff --git a/src/serializers/utils/isResourceIdentifierObject.ts b/src/serializers/utils/isResourceIdentifierObject.ts
new file mode 100644
index 0000000..868f824
--- /dev/null
+++ b/src/serializers/utils/isResourceIdentifierObject.ts
@@ -0,0 +1,34 @@
+import type { JSONAPIResourceIdentifierObject } from '../../types/resourceIdentifierObject';
+
+export const isResourceIdentifierObject = (
+  resourceIdentifierObjectCandidate: unknown,
+): resourceIdentifierObjectCandidate is JSONAPIResourceIdentifierObject => {
+  if (resourceIdentifierObjectCandidate === null) {
+    return false;
+  }
+
+  if (
+    typeof resourceIdentifierObjectCandidate !== 'object' ||
+    Array.isArray(resourceIdentifierObjectCandidate)
+  ) {
+    return false;
+  }
+
+  if ('type' in resourceIdentifierObjectCandidate === false) {
+    return false;
+  }
+
+  if (typeof resourceIdentifierObjectCandidate.type !== 'string') {
+    return false;
+  }
+
+  if ('id' in resourceIdentifierObjectCandidate === false) {
+    return false;
+  }
+
+  if (typeof resourceIdentifierObjectCandidate.id !== 'string') {
+    return false;
+  }
+
+  return true;
+};
diff --git a/src/serializers/utils/isResourceLinkage.ts b/src/serializers/utils/isResourceLinkage.ts
new file mode 100644
index 0000000..4e8e861
--- /dev/null
+++ b/src/serializers/utils/isResourceLinkage.ts
@@ -0,0 +1,17 @@
+import { isResourceIdentifierObject } from './isResourceIdentifierObject';
+
+import type { JSONAPIResourceLinkage } from '../../types/resourceLinkage';
+
+export const isResourceLinkage = (
+  resourceLinkageCandidate: unknown,
+): resourceLinkageCandidate is JSONAPIResourceLinkage => {
+  if (resourceLinkageCandidate === null) {
+    return true;
+  }
+
+  if (Array.isArray(resourceLinkageCandidate)) {
+    return resourceLinkageCandidate.every(isResourceIdentifierObject);
+  }
+
+  return isResourceIdentifierObject(resourceLinkageCandidate);
+};