Resumo. Globalization and international trade have inadvertently facilitated the spread of non-native insect species, including freshwater taxa previously considered unlikely invasive alien species. Among mayflies, only a few species have been reported as invasive, notably within the genus Cloeon Leach, 1815 (Ephemeroptera: Baetidae). This study reports the first confirmed occurrence of Cloeon smaeleni Lestage, 1924 (Ephemeroptera: Baetidae) in the state of Rio de Janeiro, southeastern Brazil, based on nymph and imago specimens collected from lentic habitats in Seropédica and Campos dos Goytacazes municipalities, in the state of Rio de Janeiro, Brazil. These new records expand the known distribution of C. smaeleni in South America and support previous predictions from species distribution models indicating its potential spread across the Neotropics. Although its introduction pathway remains uncertain, maritime transport from Africa is a plausible vector, particularly given the proximity of some sampling sites to major ports. The ecological consequences of this invasion remain unclear, but potential competition with native mayflies, especially with species of the genus Callibaetis Eaton, 1881 (Ephemeroptera: Baetidae), warrants further investigation.