An unknown green alga (order Ulvales) was a dominant macroalga in the rocky littoral zone of Lake Garda (southeastern Alps). Morphological and phylogenetic analyses indicated the alga was in the genus Jaoa, considered endemic to China. The alga in Lake Garda had numerous rhizoids, a subspherical hollow thallus with irregularly lobate—saccate morphology, and 3 layers (an outer layer of small cells with distinct chloroplasts, 2 pseudoparenchymatous, large, round, and mainly hyaline cell layers). Most characteristics were in agreement with the description of Jaoa bullata, 1 of 2 species belonging to the genus Jaoa. Phylogenetic analysis based on 3 loci (rbcL, nrDNA 18S, and ITS) placed the alga in a clade with a sample identified as Jaoa prasina from a Chinese population, but with a considerable sequence distance, a result suggesting it might be a separate species. The Lake Garda alga is a siphonaxanthin-type species that possesses only nonP lipids as structural components of chloroplast (monogalactosyl diacylglycerols, digalactosyl diacylglycerols, and sulfoquinovosyl diacylglycerols) and other cell membranes (diacylglyceryl-N,N,N-trimethylhomoserine lipids). The alga is easily detected with the naked eye, occupies a characteristic belt in the eulittoral zone, and can be found wherever hard substrata are available. Neither this species nor similar taxa are mentioned in historical (end of the 19^th century) papers on benthic algae of Lake Garda, so the alga probably was introduced to the lake in relatively recent times. This alga belongs to a little-known genus, but is easily observed and was discovered in Italy's largest lake, a tourist resource of outstanding importance. It is a striking example of the degree to which lake benthic algae are understudied in many areas.