The route proceeds to the east, meeting Interstate 495 (I-495) at exit 19 before crossing into Norfolk County and the town of Medway.

The road then crosses the Charles River as it enters Medfield, passing through the center of town where it intersects Route 27.

East of Medfield, Route 109 travels along the Dover–Walpole town line before entering Westwood.

As such, Route 109 no longer ends at a numbered highway, although the Boston-Providence Turnpike runs south from this intersection to join the current US 1 at its interchange with I-95/Route 128.

This western section of Route 109 was decommissioned in 1933, during the flooding of the Quabbin Reservoir, as it had run through the center of Enfield, Massachusetts, a town itself disincorporated in late April 1938 due to the flooding of the Quabbin Reservoir.