Brewster is twinned with the town of Budleigh Salterton in the United Kingdom.
According to the United States Census Bureau, the town has a total area of 25.4 square miles (65.9 km2), of which 22.9 square miles (59.3 km2) is land and 2.5 square miles (6.6 km2), or 10.07%, is water.
Brewster is bordered on the north by Cape Cod Bay, on the west by Dennis, on the south by Harwich, and on the east by Orleans. The town is usually separated into two villages, West and East Brewster, both of which comprise the Brewster census-designated place. Brewster is 31 miles (50 km) south of Provincetown, 14 miles (23 km) east of Barnstable, 31 miles (50 km) east of the Sagamore Bridge, and 85 miles (137 km) southeast of Boston.
The town is bordered by the Brewster Flats, an extensive stretch of tidal sand flats to the north, along the shores of Cape Cod Bay. The town is home to the Roland C. Nickerson State Forest Park, the largest state forest on Cape Cod. The town has several large ponds, especially along the Harwich town line. There are several brooks throughout the town, all of which lead to Cape Cod Bay. The bay is home to several boat landings and beaches in the town.
Brewster is home to the largest pond on Cape Cod, Long Pond. The Brewster-Harwich town line goes directly through the middle of the pond. Brewster's second largest pond is Cliff Pond, located in Nickerson State Park. Both are popular destinations.