Route 123 is a 41.91-mile-long (67.45 km) east–west state highway in southeastern Massachusetts. It crosses northern Bristol and Plymouth counties, crossing several highways along the way.

Bristol County is a county in the Commonwealth of Massachusetts. As of the 2020 census, the population was 579,200. The shire town is Taunton. Some governmental functions are performed by the Commonwealth of Massachusetts, others by the county, and others by local towns and cities.