diff --git a/lib/cast/decimal128.js b/lib/cast/decimal128.js
new file mode 100644
index 00000000000..bfb1578c406
--- /dev/null
+++ b/lib/cast/decimal128.js
@@ -0,0 +1,36 @@
+'use strict';
+
+const Decimal128Type = require('../types/decimal128');
+const assert = require('assert');
+
+module.exports = function castDecimal128(value) {
+  if (value == null) {
+    return value;
+  }
+
+  if (typeof value === 'object' && typeof value.$numberDecimal === 'string') {
+    return Decimal128Type.fromString(value.$numberDecimal);
+  }
+
+  if (value instanceof Decimal128Type) {
+    return value;
+  }
+
+  if (typeof value === 'string') {
+    return Decimal128Type.fromString(value);
+  }
+
+  if (Buffer.isBuffer(value)) {
+    return new Decimal128Type(value);
+  }
+
+  if (typeof value === 'number') {
+    return Decimal128Type.fromString(String(value));
+  }
+
+  if (typeof value.valueOf === 'function' && typeof value.valueOf() === 'string') {
+    return Decimal128Type.fromString(value.valueOf());
+  }
+
+  assert.ok(false);
+};
\ No newline at end of file
diff --git a/lib/schema/decimal128.js b/lib/schema/decimal128.js
index 38acf6ec144..0338905ffe8 100644
--- a/lib/schema/decimal128.js
+++ b/lib/schema/decimal128.js
@@ -7,7 +7,9 @@
 const SchemaType = require('../schematype');
 const CastError = SchemaType.CastError;
 const Decimal128Type = require('../types/decimal128');
+const castDecimal128 = require('../cast/decimal128');
 const utils = require('../utils');
+
 let Document;
 
 /**
@@ -37,6 +39,51 @@ Decimal128.schemaName = 'Decimal128';
 Decimal128.prototype = Object.create(SchemaType.prototype);
 Decimal128.prototype.constructor = Decimal128;
 
+/*!
+ * ignore
+ */
+
+Decimal128._cast = castDecimal128;
+
+/**
+ * Get/set the function used to cast arbitrary values to decimals.
+ *
+ * ####Example:
+ *
+ *     // Make Mongoose only refuse to cast numbers as decimal128
+ *     const original = mongoose.Schema.Types.Decimal128.cast();
+ *     mongoose.Decimal128.cast(v => {
+ *       assert.ok(typeof v !== 'number');
+ *       return original(v);
+ *     });
+ *
+ *     // Or disable casting entirely
+ *     mongoose.Decimal128.cast(false);
+ *
+ * @param {Function} [caster]
+ * @return {Function}
+ * @function get
+ * @static
+ * @api public
+ */
+
+Decimal128.cast = function cast(caster) {
+  if (arguments.length === 0) {
+    return this._cast;
+  }
+  if (caster === false) {
+    caster = v => {
+      if (v != null && !(v instanceof Decimal128Type)) {
+        throw new Error();
+      }
+      return v;
+    };
+  }
+  this._cast = caster;
+
+  return this._cast;
+};
+
 /*!
  * ignore
  */
@@ -123,35 +170,12 @@ Decimal128.prototype.cast = function(value, doc, init) {
     return ret;
   }
 
-  if (value == null) {
-    return value;
+  const _castDecimal128 = this.constructor.cast();
+  try {
+    return _castDecimal128(value);
+  } catch (error) {
+    throw new CastError('Decimal128', value, this.path);
   }
-
-  if (typeof value === 'object' && typeof value.$numberDecimal === 'string') {
-    return Decimal128Type.fromString(value.$numberDecimal);
-  }
-
-  if (value instanceof Decimal128Type) {
-    return value;
-  }
-
-  if (typeof value === 'string') {
-    return Decimal128Type.fromString(value);
-  }
-
-  if (Buffer.isBuffer(value)) {
-    return new Decimal128Type(value);
-  }
-
-  if (typeof value === 'number') {
-    return Decimal128Type.fromString(String(value));
-  }
-
-  if (typeof value.valueOf === 'function' && typeof value.valueOf() === 'string') {
-    return Decimal128Type.fromString(value.valueOf());
-  }
-
-  throw new CastError('Decimal128', value, this.path);
 };
 
 /*!
diff --git a/test/schematype.cast.test.js b/test/schematype.cast.test.js
index 743348590de..2007b52739a 100644
--- a/test/schematype.cast.test.js
+++ b/test/schematype.cast.test.js
@@ -12,6 +12,7 @@ describe('SchemaType.cast() (gh-7045)', function() {
     original.boolean = Schema.Types.Boolean.cast();
     original.string = Schema.Types.String.cast();
     original.date = Schema.Types.Date.cast();
+    original.decimal128 = Schema.Types.Decimal128.cast();
   });
 
   afterEach(function() {
@@ -19,6 +20,7 @@ describe('SchemaType.cast() (gh-7045)', function() {
     Schema.Types.Boolean.cast(original.boolean);
     Schema.Types.String.cast(original.string);
     Schema.Types.Date.cast(original.date);
+    Schema.Types.Decimal128.cast(original.decimal128);
   });
 
   it('with inheritance', function() {
@@ -165,4 +167,28 @@ describe('SchemaType.cast() (gh-7045)', function() {
       assert.throws(() => d.cast('2018-06-01'), /CastError/);
     });
   });
+
+  describe('decimal128', function() {
+    it('supports custom cast functions', function() {
+      Schema.Types.Decimal128.cast(v => {
+        assert.ok(typeof v !== 'number');
+        return original.date(v);
+      });
+
+      const d = new Schema.Types.Decimal128();
+      d.cast('1000'); // Should not throw
+
+      assert.throws(() => d.cast(1000), /CastError/);
+    });
+
+    it('supports disabling casting', function() {
+      Schema.Types.Decimal128.cast(false);
+
+      const d = new Schema.Types.Decimal128();
+      assert.throws(() => d.cast('1000'), /CastError/);
+      assert.throws(() => d.cast(1000), /CastError/);
+
+      d.cast(original.decimal128('1000')); // Should not throw
+    });
+  });
 });
\ No newline at end of file