The embankment of the Imboassica River formed the Imboassica Lagoon located to north of the State of Rio de Janeiro by the coastal sandbank, which separates it permanently from the sea. Formerly, the lagoon used to be connected to the sea naturally when waves broke the sandbar. Presently, connection is established artificially to prevent roads and houses from flooding. Such action increases the diversity of fish species living in the lagoon and therefore expands the fishing activity. This study investigated the effects of such openings on the structure of local fish community. Among the 26 species collected by gillnets, Hoplias malabaricus, Paralichythys brasiliensis, Gerres aprion, Genidens genidens, Strongylura timucu, Mugil curema, and Geophagus brasiliensis were the most abundant during the study. Lycengraulis grossidens, Archosargos probatocephalus, Tilapia rendalli, and Micropogonias furnieri were dominant before the canal was opened, whereas Anchovia clupeoides and Trachinotus carolinus were dominant in the period after the canal opening. Species diversity and evenness were practically constant during the months before the canal opening. Although both indices decreased substantially while the sea connection remained open, they returned to the same level as before when the canal was closed again. Cluster and detrended correspondence analyses showed similar patterns of two distinct groups of months before and after the canal opening supporting the idea that distinct fish assemblages occupied that environment before and after the event. Our results show that artificial canal opening is a factor that affects the structure and species composition of the fish community in the Imboassica Lagoon.