Polycheria josephensis sp. n. (Dexaminidae), an ascidian symbiont, is described from St. Joseph Bay, Florida and other locations in the Gulf of Mexico and the nearshore Atlantic Ocean from South Carolina to northern Florida. Observations on its ecology, behavior, and distribution are provided. Polycheria josephensis sp. n. is morphologically most similar to Polycheria osborni Calman, 1898 from the Pacific coast of North America. Polycheria josephensis sp. n. differs from P. osborni in the number, spacing and size of the spines on the inner plate of the maxilliped and the shape of the distal margins of coxae 1-7.