The more than two dozen species of grasses found on Assateague are vital members of the island's plant communities, acting as sediment stabilizers in both dunes and marshes. Some species are conspicuous, such as sugarcane plumegrass (Erianthus giganteus), which can grow in moist fields to a height of 12 feet. The invasive common reed (Phragmites australis), which can also reach heights of 12 feet, is an easily recognizable inhabitant of fresh water and brackish habitats, where it can out-compete many other native species. Sandburs (Cenchrus tribuloides) disperse seeds by sticking onto animal fur or clothing, and occasionally attack campers' inflatable sleeping pads. Other, less obtrusive grasses include ticklegrasses (Agrostis sp.) - short, tufted grasses which live in woods, fields, bogs, and marshes.

One of the most important grasses on the island is American beachgrass (Ammophila breviligulata), which stabilizes sand dunes and reduces erosion from wind. It continues to grow as sands blow over its stems, sometimes creating up to 40 feet of buried plant above the roots. Because of this trait, it is often planted during beach restoration projects.

Specific habitats on the island where grasses are the dominant plants include: brackish tidal marshes characterized by cattail (Typha angustifolia); marshes dominated by common reed or needlerush (Juncus roemerianus); saltwater cordgrass (Spartina alterniflora) marshes; interdunal sand bogs characterized by pathrush (Juncus dichotomus) and sundew (Drosera intermedia); and other dune, interdune, and bayside plant communities.