The sea lettuces comprise the genus Ulva, a group of edible green algae widely distributed along the coasts of the world's oceans.

The many species of sea lettuce are a popular food in many of the places where they grow, including Scandinavia, Great Britain, Ireland, China and Japan (where they are known as "aosa"). They can be eaten raw in salads, or cooked in soups, and are high in protein, soluble dietary fiber, and a variety of vitamins and minerals including especially iron.

The type species is Ulva lactuca Linnaeus.

Additionally, some species in the genus Monostroma are known as slender sea lettuces.