Plymouth Bay is a small, well-protected bay of theAtlantic Ocean on the western shore of largerCape Cod Bay along the coastline of theCommonwealth of Massachusetts. Plymouth Bay retains historical significance for the landing atPlymouth Rock in 1620 by thePilgrims aboard theMayflower who proceeded to establish the first permanentNorthern European settlement inNorth America atPlymouth Colony.

Plymouth Bay is an offshoot of the largerCape Cod Bay and is sometimes considered part ofMassachusetts Bay, which is defined byCape Ann to the north andCape Cod to the south. Plymouth Bay is also in the southern waters of theGulf of Maine in the northernAtlantic Ocean. The waters of Plymouth Bay are claimed by three Massachusetts towns,Plymouth in the southern part of the bay,Duxbury in the northern part andKingston in the westernmost extent of the bay.

Plymouth Bay is dominated by geographical features such asPlymouth Beach, a three-milebarrier beach that protectsPlymouth Harbor from the rougher seas ofCape Cod Bay. To the north, Saquish Neck serves as another barrier beach protecting the bay and is almost seven miles in length. Plymouth Beach and Saquish Neck together form a mile wide opening, connecting the bay to the open sea, with boating channels deep enough to sustain moderate boating traffic.

Within Plymouth Bay itself there are considered two smaller bays, Kingston Bay and Duxbury Bay, both in the northern waters of Plymouth Bay, andWarren Cove, in the southern waters of Plymouth Bay, located between Plymouth Beach andRocky Point.

Several peninsulas define the shoreline of Plymouth Bay such as Rocky Nook, a densely populated coastal area of Kingston, and Powder Point of Duxbury. Plymouth Bay contains one island with year-round inhabitants,Clarks Island, in the northern portion of the bay and is administered by Plymouth. Plymouth Bay also acts as the mouth for several important rivers in the region such as theJones River in Kingston and theEel River in Plymouth.

Although used for boating, Plymouth Bay itself is relatively shallow. Depths in the bay will range from 35 feet, in the deepest channels west of Fort Standish, to 6 to 42 inches throughout much of the bay's rollingmud flats.

Most of Plymouth Bay's mud flats can be found in Kingston Bay and Duxbury Bay, which are prone to becoming totally exposed in times oflow tide. The largest of these such flats is Ichabod's Flat in Kingston Bay. The mud flats of Kingston Bay are used for shellfishing andclamming and have flourished due to Plymouth Bay's ability to isolate itself fromred tide, which occasionally impacts the Massachusetts coastline.

• Plymouth Harbor
• Plymouth Sound, the bay atPlymouth inEngland.

• Map of Plymouth Bay and related bodies of water

41°58′00″N70°36′00″W / 41.96667°N 70.60000°W /41.96667; -70.60000