Contarinia geonomae Gagné, new species (Diptera: Cecidomyiidae), is described from galls found on the infructescences of Geonoma cuneata (Arecaceae) in Costa Rica. The galls are cylindrical in shape and develop concurrently with or instead of the spherical fruit. The larval chamber is located at the base of the gall, partially embedded in the infructescence. Most galls contain a single larva but some contain multiple larvae. As the gall reaches maturity the interior becomes hollow and the pupa pushes through the frayed apex just before adult emergence. Galls are most evident during the last half of the year, coincident with fruit production, but adult emergence appears to be staggered thoughout much of the year. Compared with other Contarinia, the larvae of C. geonomae are unique in having a four-pointed spatula and papillae that are nearly all devoid of setae. The pupae are unique for their prominent prothoracic spiracles.