diff --git a/lib/model.js b/lib/model.js
index a146eea1ef0..d3b995eff8e 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -3411,7 +3411,8 @@ Model.$__insertMany = function(arr, options, callback) {
   }
 
   const validationErrors = [];
-  const toExecute = arr.map(doc =>
+  const validationErrorsToOriginalOrder = new Map();
+  const toExecute = arr.map((doc, index) =>
     callback => {
       if (!(doc instanceof _this)) {
         try {
@@ -3437,6 +3438,7 @@ Model.$__insertMany = function(arr, options, callback) {
           // failed. It's up to the next function to filter out all failed models
           if (ordered === false) {
             validationErrors.push(error);
+            validationErrorsToOriginalOrder.set(error, index);
             return callback(null, null);
           }
           return callback(error);
@@ -3450,14 +3452,38 @@ Model.$__insertMany = function(arr, options, callback) {
       callback(error, null);
       return;
     }
+
+    const originalDocIndex = new Map();
+    const validDocIndexToOriginalIndex = new Map();
+    for (let i = 0; i < docs.length; ++i) {
+      originalDocIndex.set(docs[i], i);
+    }
+
     // We filter all failed pre-validations by removing nulls
     const docAttributes = docs.filter(function(doc) {
       return doc != null;
     });
+    for (let i = 0; i < docAttributes.length; ++i) {
+      validDocIndexToOriginalIndex.set(i, originalDocIndex.get(docAttributes[i]));
+    }
+
+    // Make sure validation errors are in the same order as the
+    // original documents, so if both doc1 and doc2 both fail validation,
+    // `Model.insertMany([doc1, doc2])` will always have doc1's validation
+    // error before doc2's. Re: gh-12791.
+    if (validationErrors.length > 0) {
+      validationErrors.sort((err1, err2) => {
+        return validationErrorsToOriginalOrder.get(err1) - validationErrorsToOriginalOrder.get(err2);
+      });
+    }
+
     // Quickly escape while there aren't any valid docAttributes
     if (docAttributes.length === 0) {
       if (rawResult) {
         const res = {
+          acknowledged: true,
+          insertedCount: 0,
+          insertedIds: {},
           mongoose: {
             validationErrors: validationErrors
           }
@@ -3490,6 +3516,13 @@ Model.$__insertMany = function(arr, options, callback) {
         // `insertedDocs` is a Mongoose-specific property
         const erroredIndexes = new Set((error && error.writeErrors || []).map(err => err.index));
 
+        for (let i = 0; i < error.writeErrors.length; ++i) {
+          error.writeErrors[i] = {
+            ...error.writeErrors[i],
+            index: validDocIndexToOriginalIndex.get(error.writeErrors[i].index)
+          };
+        }
+
         let firstErroredIndex = -1;
         error.insertedDocs = docAttributes.
           filter((doc, i) => {
@@ -3513,6 +3546,12 @@ Model.$__insertMany = function(arr, options, callback) {
             return doc;
           });
 
+        if (rawResult && ordered === false) {
+          error.mongoose = {
+            validationErrors: validationErrors
+          };
+        }
+
         callback(error, null);
         return;
       }
diff --git a/test/model.test.js b/test/model.test.js
index 069a5f64502..eb68df34476 100644
--- a/test/model.test.js
+++ b/test/model.test.js
@@ -4791,6 +4791,67 @@ describe('Model', function() {
       });
     });
 
+    it('insertMany() validation error with ordered false and rawResult for checking which documents failed (gh-12791)', async function() {
+      const schema = new Schema({
+        name: { type: String, required: true },
+        year: { type: Number, required: true }
+      });
+      const Movie = db.model('Movie', schema);
+
+      const id1 = new mongoose.Types.ObjectId();
+      const id2 = new mongoose.Types.ObjectId();
+      const id3 = new mongoose.Types.ObjectId();
+      const arr = [
+        { _id: id1, foo: 'The Phantom Menace', year: 1999 },
+        { _id: id2, name: 'The Force Awakens', bar: 2015 },
+        { _id: id3, name: 'The Empire Strikes Back', year: 1980 }
+      ];
+      const opts = { ordered: false, rawResult: true };
+      const res = await Movie.insertMany(arr, opts);
+      // {
+      //   acknowledged: true,
+      //   insertedCount: 1,
+      //   insertedIds: { '0': new ObjectId("63b34b062cfe38622738e510") },
+      //   mongoose: { validationErrors: [ [Error], [Error] ] }
+      // }
+      assert.equal(res.insertedCount, 1);
+      assert.equal(res.insertedIds[0].toHexString(), id3.toHexString());
+      assert.equal(res.mongoose.validationErrors.length, 2);
+      assert.ok(res.mongoose.validationErrors[0].errors['name']);
+      assert.ok(!res.mongoose.validationErrors[0].errors['year']);
+      assert.ok(res.mongoose.validationErrors[1].errors['year']);
+      assert.ok(!res.mongoose.validationErrors[1].errors['name']);
+    });
+
+    it('insertMany() validation error with ordered false and rawResult for mixed write and validation error (gh-12791)', async function() {
+      const schema = new Schema({
+        name: { type: String, required: true, unique: true },
+        year: { type: Number, required: true }
+      });
+      const Movie = db.model('Movie', schema);
+      await Movie.init();
+
+      const arr = [
+        { foo: 'The Phantom Menace', year: 1999 },
+        { name: 'The Force Awakens', bar: 2015 },
+        { name: 'The Empire Strikes Back', year: 1980 },
+        { name: 'The Empire Strikes Back', year: 1980 }
+      ];
+      const opts = { ordered: false, rawResult: true };
+      const err = await Movie.insertMany(arr, opts).then(() => null, err => err);
+
+      assert.ok(err);
+      assert.equal(err.insertedDocs.length, 1);
+      assert.equal(err.insertedDocs[0].name, 'The Empire Strikes Back');
+      assert.equal(err.writeErrors.length, 1);
+      assert.equal(err.writeErrors[0].index, 3);
+      assert.equal(err.mongoose.validationErrors.length, 2);
+      assert.ok(err.mongoose.validationErrors[0].errors['name']);
+      assert.ok(!err.mongoose.validationErrors[0].errors['year']);
+      assert.ok(err.mongoose.validationErrors[1].errors['year']);
+      assert.ok(!err.mongoose.validationErrors[1].errors['name']);
+    });
+
     it('insertMany() populate option (gh-9720)', async function() {
       const schema = new Schema({
         name: { type: String, required: true }