Sea Lettuce is thin flat green algae that normally may reach 45 cm or more in length, though generally much less, and up to 30 centimetres (12 in) across. The plant is found growing attached to cobbles, boulders and bedrock in protected and semi-protected habitats, moderately exposed rocks and shores, in pools and shallow waters near the low watermark, brackish waters and other surfaces using a small holdfast or living in rock pools if it has become detached. The detached fronds continue to grow and can create large floating colonies. It thrives in brackish waters, particularly in waters with organic enrichment.

The plant is composed of lobed, ruffle-edged leaves that are coarse and sheet-like and resemble a leaf of lettuce. The leaves may appear flat, thin, broad, and often rounded or oval. Its leaves are often perforated with holes of various sizes. Almost no stalk exists at the point of attachment, and no true roots are present.