Plymouth is located at 41°57'29" North, 70°40'4" West (41.958056, -70.667778).
According to the United States Census Bureau, the town has a total area of 3.91 mi2 (10.12 km2). 2.26 mi2 (5.85 km2) of it is land and 1.65 mi2 (4.27 km2) of it is covered by water. The total area is 42.19% water.
Plymouth is located in the Eastern Standard Time (EST) timezone, GMT-05:00. It observes Daylight Saving Time (DST).