State Route 24 is part of Maine's system of numbered state highways, running from Bailey Island south of Harpswell northward to Gardiner, passing through Brunswick along the way.

State Route 24 begins at the southern tip of Bailey Island in the town of Harpswell, where it dead-ends. It proceeds northward through the "downtown" area of Harpswell on Orr's Island and continues onto mainland Maine where it crosses into the town of Brunswick. SR 24 continues north until reaching an interchange with U.S. Route 1, then turns west towards downtown. In downtown Brunswick, SR 24 meets the northern terminus of SR 123 before turning north onto Maine Street at Bowdoin College. SR 24 continues through the heart of town and crosses underneath US 1 at another interchange. At this point, U.S. Route 201 begins overlapped with SR 24 and both routes cross the Androscoggin River into Topsham. SR 24 splits off from US 201, crosses SR 196 and enters the town of Bowdoinham where it meets the northern terminus of SR 125 and turns northeast. SR 24 enters Richmond and briefly overlaps with SR 197 before continuing into Gardiner, paralleling the river at this point. SR 24 comes to an end in the heart of Gardiner at the intersection of Maine Avenue and Bridge Street (US 201, SR 9, SR 27, and SR 126).

SR 24 between its southern terminus and the Androscoggin River crossing was formerly designated SR 124. It was renumbered to 24 in the early 1930s when the number 124 was reassigned to a completely new route. From Topsham, SR 24 was designated over a new routing to its current northern terminus in Gardiner and ultimately as far as Guilford.

From Gardiner, SR 24 used to run concurrently with US 201/SR 27 to Sidney, a designation which was simply dropped following SR 24's truncation in 1954. The remainder of the route, which runs from Sidney to Guilford, was renumbered SR 23. This 1954 routing remains to the present.