A male specimen of Tripedalia cystophora (Cubozoa: Tripedaliidae) was collected from Lake Wyman, Boca Raton, Florida, USA. This is the first report of this species from the continental United States and brings the total known number of cubozoan species living in this region to four. Lake Wyman is a natural lagoon/estuary ecosystem which is part of the Atlantic Intracoastal Waterway. The box jellyfish was found in shallow water around the roots of the red mangrove, Rhizophora mangle, where it was observed feeding on copepods attracted to light. This finding may indicate a local population in the waters of south Florida, USA, but an isolated occurrence cannot be ruled out.