A new species of Petasiger inhabits Pelecanus occidentalis, from the Area de Conservación Guanacaste, Costa Rica. The new species most closely resembles Petasiger novemdecim Lutz 1929 and Petasiger caribbensis Nassi, 1980 by having 19 circumoral spines and vitelline follicles confluent between the ventral sucker and gonads. The new species differs from both of these species by having a cirrus sac that is anteroposteriorly elongate and that reaches posteriorly to the midventral sucker, an ovary and Mehlis' gland that overlap the anterior testis dorsally, a uterus that lies dorsal to the ventral sucker, and a dextromedial genital pore. In P. novemdecim and P. caribbensis the cirrus sac is round and does not extend posteriorly to the anterior margin of the ventral sucker, both the ovary and Mehlis' gland are anterior to the anterior testis, the uterus runs lateral to and not dorsal to the ventral sucker, and the genital pore opens medially and sinistromedially, respectively.