Bopyrione synalphei, new genus, new species, a parasite of Synalpheus goodei Coutière in the northeastern Gulf of Mexico, S. bousfieldi Chace, S. pectiniger Coutière and Synalpheus spp. in Haiti and Synalpheus sp. in Curaçao, is described. A member of the bopyrid subfamily Bopyrinae, whose diagnosis is slightly amended to accommodate it, it is closely allied to the genera Bopyroides Stimpson and Bopyrina Kossmann. Reexamination of the holotype of Bopyroides woodmasoni Chopra, 1923, a parasite of Synalpheus neomeris (de Man) in the Andaman Islands, indicates that it is also assignable to Bopyrione.