A new species, Ophiothrix synoecina, is described and figured. O. synoecina is an obligate associate of the regular sea urchin Echinometra lucunter (L.) in the area of Santa Marta on the Caribbean coast of Colombia. The rock-boring echinometrid E. lucunter inhabits heavily exposed areas of rocky shores. O. synoecina lives in the boreholes excavated by the sea urchin, spending its life under the echinoid.