State Route 123 (SR 123) is part of Maine's system of numbered state highways, running from Harpswell to SR 24 Business in Brunswick.

SR 123 begins at the intersection of Potts Point Road and Evergreen Avenue near the southernmost point of Harpswell Neck within the town of Harpswell.

As it continues north, the density of houses lining the road decreases as it heads through Harpswell Center and North Harpswell.

After crossing into Brunswick, the road exits the Harpswell Neck peninsula and passes through the small settlement of Dyers Corner.

Before the highway ends, SR 123 heads around the east side of Bowdoin College.