The Thomaston CDP is slightly east of the geographic center of the town of Thomaston, on the west side of the
Naugatuck River and its valley. It extends to the south as far as Watertown Road, to the west beyond Hillside Cemetery and to Northfield Brook, to the north to a power line south of D. Welton Way, and to the east to Williams Street, Electric Avenue, and the Naugatuck River.
U.S. Route 6 follows Main Street through Thomaston; it leads east to
Bristol and southwest to
Watertown. The
Connecticut Route 8 freeway passes east of the CDP, leading south to
Waterbury. According to the
U.S. Census Bureau, the Thomaston CDP has a total area of , of which , or 0.95%, are water. ==Demographics==