Topsham is a town in Sagadahoc County, Maine, United States. It is part of the PortlandSouth PortlandBiddeford, Maine metropolitan statistical area. The town is home to the annual Topsham Fair. According to the United States Census Bureau, the town has a total area of 35.4 square miles (91.8 km²), of which, 32.0 square miles (82.9 km²) of it is land and 3.4 square miles (8.8 km²) of it (9.62%) is water. Located beside Merrymeeting Bay at the mouth of the Androscoggin River, Topsham is drained by the Cathance River and Muddy River.