We report here the 98.5 Mbp haploid genome (12,924 protein coding genes) of Ulva mutabilis, a ubiquitous and iconic representative of the Ulvophyceae or green seaweeds. Ulva's rapid and abundant growth makes it a key contributor to coastal biogeochemical cycles; its role in marine sulfur cycles is particularly important because it produces high levels of dimethylsulfoniopropionate (DMSP), the main precursor of volatile dimethyl sulfide (DMS). Rapid growth makes Ulva attractive biomass feedstock but also increasingly a driver of nuisance “green tides.” Ulvophytes are key to understanding the evolution of multicellularity in the green lineage, and Ulva morphogenesis is dependent on bacterial signals, making it an important species with which to study cross-kingdom communication. Our sequenced genome informs these aspects of ulvophyte cell biology, physiology, and ecology. Gene family expansions associated with multicellularity are distinct from those of freshwater algae. Candidate genes, including some that arose following horizontal gene transfer from chromalveolates, are present for the transport and metabolism of DMSP. The Ulva genome offers, therefore, new opportunities to understand coastal and marine ecosystems and the fundamental evolution of the green lineage. De Clerck et al. present the first genome sequence of a green seaweed, a dominant group of primary producers in coastal environments. The Ulva genome informs on an independent acquisition of multicellularity, sheds light on adaptations to life in intertidal habitats, and identifies candidate genes involved in DMSP biosynthesis and conversion to DMS.