This paper describes the interspecific fostering of two Ring-billed Gull chicks (Larus delawarensis) following natural adoption by a pair of Caspian Terns (Hydroprogne caspia) at Gull Island, Presqu'ile Provincial Park, Ontario, Canada, in 2011. Naturally-occurring, interspecific adoptions among waterbirds are rare and those between distantly-related species have not been confirmed to produce fledglings. The observation of one Ring-billed Gull chick being raised by Caspian Terns beyond fledging (approximately 46 days of age) appears to be the first record of successful adoption between different taxonomic families of waterbirds. Behavior, provisioning rate, and growth of chicks were recorded to document this adoption. During linear growth, the mass of the adopted Ring-billed Gull chicks were close to, or even exceeding, maximum mass of Ring-billed Gull chicks raised by their natural parents. After this period, however, the adoptive Caspian Tern parents reduced provisioning rates below those recorded for conspecifics and the adopted Ring-billed Gull chicks lost mass, one of which left the territory prematurely and died. Although initial circumstances leading to the adoption were unclear, that one Ring-billed Gull chick was ultimately fledged by Caspian Terns is remarkable given the different provisioning methods and diet of these species.