State Highway 303 followed Thompsonville Road in Suffield, the Thompsonville Bridge across the Connecticut River, and Main Street in Enfield, ending at US 5.

In 1932, SH 303 became part of the original alignment of Route 190. In 1967, a new Connecticut River bridge opened to the south, and the former SH 303 became unposted SR 590. Within a few years, SR 590 was turned over to the towns, and the Thompsonville Bridge was demolished.