Ulvalactuca, also known by the common name sea lettuce, is an edible green alga in the family Ulvaceae. It is the type species of the genus Ulva. A synonym is U. fenestrata, referring to its “windowed” or “holed” appearance.

Ulvalactuca is a thin flat green algae growing from a discoid holdfast. The margin is somewhat ruffled and often torn. It may reach 18 centimetres (7.1 in) or more in length, though generally much less, and up to 30 centimetres (12 in) across. The membrane is two cells thick, soft and translucent, and grows attached, without a stipe, to rocks or other algae by a small disc-shaped holdfast.

Green to dark green in colour, this species in the Chlorophyta is formed of two layers of cells irregularly arranged, as seen in cross-section. The chloroplast is cup-shaped in some references but as a parietal plate in others with one to three pyrenoids. There are other species of Ulva which are similar and not always easy to differentiate.