Thalli thin, sheet-like, consisting of wide blades, 10 - 15 cm wide at base, tapering upward to less that 2.5 cm wide at tip. Up to 1 meter long. Basally broadened, but upper portions divided deeply into many ribbon like segments; margins smooth, often undulate. Holdfast is small without dark rhizoids.  Bright grass green to dark green, gold at margins when reproductive. May be colorless when stressed.

U. rigida is similar but has tough dark rhizoids, smaller, more rectangular cells, and separates easily into two sheets.

Ulva fasciata is commonly found on intertidal rocks, in tidepools, and on reef flats. Often abundant in areas of fresh water runoff high in nutrients such as near the mouth of streams and run-off pipes.

Ulva fasciata, or "sea lettuce", is commonly found in areas where nutrients are high, wave forces low and herbivory reduced. It is tolerant of stressful conditions, and its presence often indicates freshwater input or pollution.

Ulva species are early-successional algae, quickly taking over new substrate on boulders that are cleared by storm disturbance. U. fasciata and Enteromorpha flexuosa are generally the first macroalgae to colonize newly opened substrate in intertidal areas with high nutrients. Their opportunistic success can be attributed to their simple morphologies and fecundity. In Ulva species, between 20 to 60 percent of their overall biomass can be allocated monthly to reproduction. The alga’s reproductive success is partly due to the reproductive cells’ photosynthetic ability. The zoospores’ and gametes’ ability to photosynthesize subsidizes their motility and rapid growth once attached to the substrate. Reproductive cells of U. fasciata have similar photosynthetic rates to adult vegetative cells, with higher respiration rates.

Ecologically successful green alga like U. fasciata are potentially invasive. Coastal waters near harbors, industrial complexes and residential areas with nutrient- rich and/or fresh water input often have blooms of Ulva species that coat ships’ hulls, cover pilings and shorelines, and restrict outflow pipes. U. fasciata is classified as a marine fouling organism, and studies in control and eradication are presently underway.