We report the first observations of cooperative polyandry in the Red-footed Booby (Sula sula), a trio that remained stable over at least 4 consecutive years. Both males were heterosexual; neither was dominant. One male was more sexually active, but mounting activity suggested shared paternity over the years. Trio males, compared to monogamous males, invested more in pair bond maintenance activity which possibly assisted in maintaining a stable breeding unit. Trio members, like monogamous adults, shared breeding duties evenly, reducing the total investment for each trio member relative to that of monogamous adults. The extra foraging effort of the trio enabled similar provisioning rates in good and bad years. We suggest trio formation improved female lifetime reproductive success due to enhanced nest, egg, and chick care; maintained stable food provisioning rates despite highly heterogeneous foraging conditions; and increased male survivorship due to sharing of breeding duties. Red-footed Booby trio formation is probably an “opportunistic deviation” from normal monogamy which is favored by differential costs and benefits to males and females.