Bonytail Gila elegans, a large‐bodied cyprinid that is endemic to the Colorado River basin of the American Southwest, was historically widespread and abundant in large warmwater streams but is now critically endangered. To increase recovery prospects, over 500,000 Bonytails have been stocked in the upper Colorado River basin since 2000, but adult survival has been low and reproduction has not been detected. We provide the first documented evidence of successful reproduction by stocked Bonytails in the upper Colorado River basin. Adult Bonytails were stocked in the Green River and accessed Stewart Lake and Johnson Bottom (managed floodplain wetlands in the middle Green River, Utah) during high flows in May 2015 (Stewart Lake only) and 2016. Draining of Stewart Lake in September 2015 revealed 19 age‐0 individuals of Gila sp. (37–64 mm TL) among over 405,000 fish. Four preserved specimens (41–48 mm TL) were verified as Bonytails by using morphological and molecular techniques. Otolith daily increment analysis confirmed reproduction by Bonytails in Stewart Lake. Bonytail reproduction was also noted during 2016 in Stewart Lake (probable) and Johnson Bottom. Young Bonytails survived despite the presence of abundant nonnative fish predators. Use of floodplain wetlands for reproduction may enhance the recovery of critically endangered Bonytail in the upper Colorado River basin. Received September 1, 2016; accepted January 2, 2017 Published online March 15, 2017