Cetacea
- Fin Whale Balaenoptera physalus (SC) - Formerly common offshore. Now its seldom seen offshore. Stranded in Barnstable county, Essex county, Dukes county, and Plymouth county. Endangered in Massachusetts. It is now found mostly in Cape Cod Bay, were its seen breaching. The second largest animal in the world, just after the Blue Whale. Growing to lengths of 85 feet, and weighing 80 to 90 tons.
- Blue Whale Balaenoptera musculus (SC) - One questionable stranding in Essex county in 1755. Recent near-shore records. Endangered in Massachusetts. Although it is the largest animal in existence, populations of the Northern Hemisphere grow 89 to 94 feet and weigh 105 tons, while populations in the Southern Hemisphere grow 100 feet and weigh 150 to 200 tons.
- Sei Whale Balaenoptera borealis (SC) - Found in Cape Cod bay and Massachusetts Bay. Stranded in Barnstable (1910 and 1974), Essex (2007), and Plymouth county (1948). Endangered in Massachusetts. Third largest Roqual, just after the Fin Whale and Blue Whale. Although its mistaken for the Bryde's Whale (which is not found in the commonwealth), it is slightly larger and heavier, and is found in cold waters, while the smaller Bryde's Whale is found near tropical and warm waters.
- Minke Whale Balaenoptera acutorostrata - The smallest of the Roquals, and the second smallest Baleen whale, just after the Pygmy right whale. Found in the inshore waters. Stranded in Norfolk, Essex, Barnstable, Dukes, and Nantucket county. The only Roqual in New England that is not threatened. Also the most common Roqual in Massachusetts.
- Humpback Whale Megaptera novaeangliae (SC) - The most well known of the Roquals. Stranded in Barnstable, Dukes, Essex, Nantucket, Norfolk, Plymouth, and Suffolk county. In Cape Cod bay, it is seen breaching.
- Gray Whale Eschrichtius robustus (EX) - Probably found in offshore waters. Whaling and overhunting has caused the Gray Whale to become extinct from the North Atlantic. There may be some reintroductions if the population in the Pacific starts rising.
- North Atlantic Right Whale Eubalaena glacialis (SC) - One of the rarest whales in the world. Originally found from New England to Florida, and as far east as West Africa. Because of overhunting, it has become extinct from the East Atlantic. Formerly stranded in the commonwealth. Recorded from Plymouth, Essex, Barnstable, Dukes, and Nantucket county. It is endangered in Massachusetts. There are only 350 North Atlantic Right Whales in the world.
- Sperm Whale Physeter macrocephalus (SC) - Formerly abundant offshore. Stranded in Barnstable, Dukes, Essex, Nantucket, and Plymouth county. The largest toothed whale. Dives deep to hunt for food.
- Pygmy Sperm Whale Kogia breviceps
- Dwarf Sperm Whale Kogia sima
- Northern Bottlenose Whale Hyperoodon ampullatus
- Sowerby's Beaked Whale Mesplodon bidens
- Blainville's Beaked Whale Mesplodon densirostris
- Gervais' Beaked Whale Mesplodon europaeus (A)
- True's Beaked Whale Mesplodon mirus
- Cuvier's Beaked Whale Ziphius cavirostris
- Beluga Delphinapterus leucas (A)
- Short-beaked Common Dolphin Delphinus delphis
- Long-finned Pilot Whale Globicephala melas
- Risso's Dolphin Grampus griseus
- Atlantic White-sided Dolphin Lagenorhynchus acutus
- White-beaked Dolphin Lagenorhynchus albirostris
- Killer Whale Orcinus orca
- False Killer Whale Pseudorca crassidens (A)
- Striped Dolphin Stenella coeruleoalba (A)
- Atlantic Spotted Dolphin Stenella fontalis (A)
- Bottlenose Dolphin Tursiops truncatus
- Harbor Porpoise Phocoena phocoena
Read more about this topic: List Of Mammals Of Massachusetts