State Route 178 (SR 178) is part of Maine's system of numbered state highways, located along the east side of the Penobscot River northeast of Bangor.

It runs from State Street in Brewer (where it is concurrent with SR 9) north to Milford where it terminates at U.S Route 2 (US 2) just east of Old Town.

The two routes proceed concurrently along the Penobscot River for four miles (6.4 km) towards Eddington.

SR 178 continues northward along the river, passing through the town of Bradley before reaching Milford where it terminates at US 2 (which crosses the river from Old Town, just to the west).

State Street was, in the past, part of a numbered highway - US 1A Business.