diff --git a/packages/-ember-data/tests/unit/adapters/rest-adapter/fetch-options-test.js b/packages/-ember-data/tests/unit/adapters/rest-adapter/fetch-options-test.js
new file mode 100644
index 00000000000..07cdb6a3e1d
--- /dev/null
+++ b/packages/-ember-data/tests/unit/adapters/rest-adapter/fetch-options-test.js
@@ -0,0 +1,148 @@
+import { module, test } from 'qunit';
+import { fetchOptions } from '@ember-data/adapter/rest';
+
+module('unit/adapters/rest-adapter/fetch-options', function(hooks) {
+  test("fetchOptions removes undefined query params when method is POST and 'data' is an object", function(assert) {
+    assert.expect(1);
+
+    const dataAsObject = {
+      a: 1,
+      b: undefined,
+      c: 3,
+      d: null,
+      e: 0,
+      f: false,
+    };
+
+    const undefinedQueryStringOptions = {
+      url: 'https://emberjs.com',
+      method: 'POST',
+      data: dataAsObject,
+    };
+
+    let options = fetchOptions(undefinedQueryStringOptions);
+    assert.deepEqual(options.body, '{"a":1,"c":3,"d":null,"e":0,"f":false}');
+  });
+
+  test('fetchOptions sets the request body correctly when the method is not GET or HEAD', function(assert) {
+    assert.expect(3);
+
+    const baseOptions = {
+      url: '/',
+      method: 'POST',
+      data: { a: 1 },
+    };
+
+    // Tests POST method.
+    let options = fetchOptions(baseOptions);
+    assert.equal(options.body, JSON.stringify(baseOptions.data), 'POST request body correctly set');
+
+    // Tests PUT method.
+    baseOptions.method = 'PUT';
+    options = fetchOptions(baseOptions);
+    assert.equal(options.body, JSON.stringify(baseOptions.data), 'PUT request body correctly set');
+
+    // Tests DELETE method.
+    baseOptions.method = 'DELETE';
+    options = fetchOptions(baseOptions);
+    assert.equal(options.body, JSON.stringify(baseOptions.data), 'DELETE request has the correct body');
+  });
+
+  test("fetchOptions sets the request body correctly when the method is POST and 'data' is a string", function(assert) {
+    assert.expect(1);
+
+    // Tests stringified objects.
+    const stringifiedData = JSON.stringify({ a: 1, b: 2 });
+    const optionsWithStringData = {
+      url: 'https://emberjs.com',
+      method: 'POST',
+      data: stringifiedData,
+    };
+
+    let options = fetchOptions(optionsWithStringData);
+    assert.equal(options.body, stringifiedData);
+  });
+
+  test('fetchOptions does not set a request body when the method is GET or HEAD', function(assert) {
+    assert.expect(4);
+
+    const baseOptions = {
+      url: '/',
+      method: 'GET',
+      data: { a: 1 },
+    };
+
+    let options = fetchOptions(baseOptions);
+    assert.strictEqual(options.body, undefined, 'GET request does not have a request body');
+
+    baseOptions.method = 'HEAD';
+    options = fetchOptions(baseOptions);
+    assert.strictEqual(options.body, undefined, 'HEAD request does not have a request body');
+
+    baseOptions.data = {};
+    options = fetchOptions(baseOptions);
+    assert.strictEqual(
+      options.body,
+      undefined,
+      'HEAD request does not have a request body when `data` is an empty object'
+    );
+
+    baseOptions.method = 'GET';
+    options = fetchOptions(baseOptions);
+    assert.strictEqual(
+      options.body,
+      undefined,
+      'GET request does not have a request body when `data` is an empty object'
+    );
+  });
+
+  test("fetchOptions correctly processes an empty 'data' object", function(assert) {
+    assert.expect(2);
+
+    const getData = {
+      url: 'https://emberjs.com',
+      method: 'GET',
+      data: {},
+    };
+
+    const getOptions = fetchOptions(getData);
+    assert.equal(getOptions.url.indexOf('?'), -1, 'A question mark is not added if there are no query params to add');
+
+    const postData = {
+      url: 'https://emberjs.com',
+      method: 'POST',
+      data: {},
+    };
+
+    const postOptions = fetchOptions(postData);
+    assert.equal(postOptions.body, '{}', "'options.body' is an empty object");
+  });
+
+  test("fetchOptions sets the request body correctly when 'data' is FormData", function(assert) {
+    assert.expect(1);
+
+    const formData = new FormData();
+    const postData = {
+      url: 'https://emberjs.com',
+      method: 'POST',
+      data: formData,
+    };
+
+    const postOptions = fetchOptions(postData);
+    assert.strictEqual(postOptions.body, formData, "'options.body' is the FormData passed in");
+  });
+
+  test("fetchOptions sets the request body correctly when 'data' is a String", function(assert) {
+    assert.expect(1);
+
+    let stringBody = JSON.stringify({ a: 1, b: 2, c: 3 });
+    const postData = {
+      url: 'https://emberjs.com',
+      method: 'POST',
+      data: stringBody,
+    };
+
+    const postOptions = fetchOptions(postData);
+    assert.equal(postOptions.body, stringBody, "'options.body' is the String passed in");
+  });
+});
diff --git a/packages/adapter/addon/rest.js b/packages/adapter/addon/rest.js
index 9309d8ad4db..4e67d528354 100644
--- a/packages/adapter/addon/rest.js
+++ b/packages/adapter/addon/rest.js
@@ -1363,7 +1363,19 @@ export function fetchOptions(options, adapter) {
     } else {
       // NOTE: a request's body cannot be an object, so we stringify it if it is.
       // JSON.stringify removes keys with values of `undefined` (mimics jQuery.ajax).
-      options.body = JSON.stringify(options.data);
+      // If the data is not a POJO (it's a String, FormData, etc), we just set it.
+      // If the data is a string, we assume it's a stringified object.
+
+      /* We check for Objects this way because we want the logic inside the consequent to run
+       * if `options.data` is a POJO, not if it is a data structure whose `typeof` returns "object"
+       * when it's not (Array, FormData, etc). The reason we don't use `options.data.constructor`
+       * to check is in case `data` is an object with no prototype (e.g. created with null).
+       */
+      if (Object.prototype.toString.call(options.data) === '[object Object]') {
+        options.body = JSON.stringify(options.data);
+      } else {
+        options.body = options.data;
+      }
     }
   }