During a genetic analysis of western Atlantic Wrangeliaceae J.Agardh, specimens closely related to the generitype of Lophothamnion J.Agardh fell into a clade including the generitype of the earlier described Pleonosporium Nägeli, causing us to subsume the former genus. Two new species are described, P. novae-angliae G.W.Saunders & C.W.Schneider, sp. nov. for specimens from southern New England and New York, United States, formerly identified there as P. borreri (Smith) Nägeli, and P. ricksearlesii C.W.Schneider & G.W.Saunders, sp. nov. from Bermuda. The relationship of Pleonosporium with its sister genus Spongoclonium Sonder is discussed with the transfer of Spongoclonium australicum Womersley to Pleonosporium.