Lempholemma syreniarum is described from Ontario, Canada in North America. The new species grows on the bark of deciduous tree bases that are seasonally flooded. In seasonally dry periods, its black areolate thallus, typically with numerous light brown apothecia, is easily seen on tree bases and root flares but is underwater during most other parts of the year. A key is given aiding identification of small crustose-squamulose and corticolous cyanolichens with simple ascospores.