Route 40 is a 10.46-mile-long (16.83 km) east–west state route located in northeastern Massachusetts, entirely within Middlesex County.

The short highway connects Groton and Chelmsford.

The highway proceeds eastward out of the downtown area, passing through the northern part of the town of Westford.

Most of the route winds through rather densely wooded areas, with several ponds located alongside or near the highway.

Route 40 continues another mile eastward, emerging from the woods before ending at an intersection with Route 3A in the center of North Chelmsford at Vinal Square, located just west of the Merrimack River.