diff --git a/lib/collection.js b/lib/collection.js
index e5d457c..09e6384 100644
--- a/lib/collection.js
+++ b/lib/collection.js
@@ -290,7 +290,76 @@ module.exports = function Collection(db, state) {
       }
     },
     rename: NotImplemented,
-    replaceOne: NotImplemented,
+    replaceOne: function (selector, data, options, callback) {
+      callback = arguments[arguments.length - 1];
+      if (typeof options !== 'object') options = {};
+
+      var opResult = {
+        result: {
+          ok: 1,
+          nModified: 0,
+          n: 0
+        },
+        connection: db,
+        matchedCount: 0,
+        modifiedCount: 0,
+        upsertedCount: 0,
+        upsertedId: null
+      };
+      var action = (options.upsert ? "upsert: " : "replace: ");
+      debug('%s.%s %j', name, action, selector);
+
+      asyncish(function () {
+        var docs = first(selector, state.documents || []) || [];
+        if (!Array.isArray(docs)) docs = [docs];
+        debug('%s.%s %j', name, action, docs);
+
+
+        if(!docs.length && options.upsert) {
+          var cloneData = upsertClone(selector, data);
+          var cloned = _.cloneDeepWith(cloneData, cloneObjectIDs);
+          cloned._id = cloned._id || pk();
+
+          debug('%s.%s checking for index conflict', name, action);
+          var conflict = state.findConflict(cloned);
+          if (conflict) {
+            debug('conflict found %j', conflict);
+            return callback(conflict);
+          }
+
+          if (!state.documents) state.documents = [cloned];
+          else state.documents.push(cloned);
+
+          opResult.matchedCount = opResult.result.n = 1;
+          opResult.upsertedCount = opResult.result.nModified = 1;
+          opResult.upsertedId = { _id: cloned._id };
+        }
+        else if (docs.length > 0) {
+          debug('%s.%s modifying doc', name, action);
+          debug('%s.%s %j', name, action, docs);
+          Object.keys(docs[0]).forEach(function (k) {
+            if (k === "_id") {
+              return;
+            }
+            _.unset(docs[0], k);
+          });
+          modify(docs[0], data, state);
+          debug('%s.%s %j', name, action, docs);
+          opResult.matchedCount = opResult.result.n = docs.length;
+          opResult.modifiedCount = opResult.result.nModified = docs.length;
+          opResult.upsertedId = docs[0]._id;
+        }
+
+        state.persist();
+        callback(null, opResult);
+      });
+
+      if (typeof callback !== 'function') {
+        return new Promise(function (resolve, reject) {
+          callback = function (e, r) { e ? reject(e) : resolve(r) };
+        })
+      }
+    },
     save: function (doc, options, callback) {
       callback = arguments[arguments.length-1];
       if(typeof options!=='object') options = {};
diff --git a/test/mock.test.js b/test/mock.test.js
index e55cdde..6ff1c27 100644
--- a/test/mock.test.js
+++ b/test/mock.test.js
@@ -342,6 +342,99 @@ describe('mock tests', function () {
       });
     })
 
+    it('should NOT replace one (replaceOne) with no document found', function (done) {
+      //query, data, options, callback
+      collection.replaceOne({ test: 1456 }, { test: 1456, b: false, alsoDeep: { deepC: 'C' } }, function (err, opResult) {
+        if(err) return done(err);
+        opResult.result.n.should.equal(0);
+
+        collection.findOne({ test:1456 }, function (err, doc) {
+          if(err) return done(err);
+          (!!doc).should.be.false;
+          done();
+        });
+      });
+    });
+
+    it('should replace one (replaceOne) with matching document found', function (done) {
+      //query, data, options, callback
+      collection.insertOne({ test: 1457, a: true, deep: { bar: "buzz", fang: "dang" } }, function (insertErr, insertResult) {
+        if(insertErr) return done(insertErr);
+        insertResult.result.n.should.equal(1);
+
+        collection.replaceOne({ test: 1457 }, { test: 1457, b: false, alsoDeep: { deepC: 'C' } }, function (err, opResult) {
+          if(err) return done(err);
+          opResult.result.n.should.equal(1);
+
+          collection.findOne({ test: 1457 }, function (err, doc) {
+            if(err) return done(err);
+            (!!doc).should.be.true;
+            doc.should.not.have.property('a');
+            doc.should.not.have.property('deep');
+            doc.should.have.property('b', false);
+            doc.should.have.property('alsoDeep');
+            doc.alsoDeep.should.have.property('deepC', 'C');
+
+            collection.remove({ test: 1457 }, function (errRemove) {
+              if (errRemove) return done(errRemove);
+              done();
+            });
+          });
+        });
+      });
+    });
+
+    it('should replace one (replaceOne) with upsert and matching document found', function (done) {
+      //query, data, options, callback
+      collection.insertOne({ test:1458, a: true, deep: { bar: "buzz", fang: "dang" } }, function (insertErr, insertResult) {
+        if(insertErr) return done(insertErr);
+        insertResult.result.n.should.equal(1);
+
+        collection.replaceOne({ test: 1458 }, { test: 1458, b: false, alsoDeep: { deepC: 'C' } }, { upsert: true }, function (err, opResult) {
+          if(err) return done(err);
+          opResult.result.n.should.equal(1);
+
+          collection.findOne({ test: 1458 }, function (err, doc) {
+            if(err) return done(err);
+            (!!doc).should.be.true;
+            doc.should.not.have.property('a');
+            doc.should.not.have.property('deep');
+            doc.should.have.property('b', false);
+            doc.should.have.property('alsoDeep');
+            doc.alsoDeep.should.have.property('deepC', 'C');
+
+            collection.remove({ test: 1458 }, function (errRemove) {
+              if (errRemove) return done(errRemove);
+              done();
+            });
+          });
+        });
+      });
+    });
+
+    it('should replace one (replaceOne) with upsert and no document found', function (done) {
+      //query, data, options, callback
+      collection.replaceOne({ test: 1459 }, { test: 1459, b: false, alsoDeep: { deepC: 'C' } }, { upsert: true }, function (err, opResult) {
+        if(err) return done(err);
+        opResult.result.n.should.equal(1);
+
+        collection.findOne({ test:1459 }, function (err, doc) {
+          if(err) return done(err);
+          (!!doc).should.be.true;
+          doc.should.not.have.property('a');
+          doc.should.not.have.property('deep');
+          doc.should.have.property('b', false);
+          doc.should.have.property('alsoDeep');
+          doc.alsoDeep.should.have.property('deepC', 'C');
+
+          collection.remove({ test: 1459 }, function (errRemove) {
+            if (errRemove) return done(errRemove);
+            done();
+          });
+        });
+      });
+    });
+
     it('should update one (updateOne)', function (done) {
       //query, data, options, callback
       collection.updateOne({test:123}, { $set: { foo: { bar: "buzz", fang: "dang" } } }, function (err, opResult) {