Deinosuchus is a genus of large crocodylian that inhabited North America during the Late Cretaceous. This massive predator has become one of the most well-known prehistoric organisms, with a considerable amount of literature on its biogeography, ecology, and evolution published. However, ontogenetic changes of Deinosuchus and other species of extinct large, predatory crocodyliforms have remained poorly understood because of a lack of remains known from juvenile individuals and issues surrounding the ability of histological analysis of adult material to provide information on yearly growth. Here, I describe a tooth from a juvenile Deinosuchus estimated at less than 1m in total body length. As the first reported specimen of a juvenile Deinosuchus, to the author's knowledge, in the literature, the new fossil evinces the extremely small size of young individuals of this taxon compared to adults more than 8 m and 3,500 kg. Furthermore, the tooth shows that some morphological discrepancies existed between the dentition of juvenile and adult Deinosuchus individuals, including the size of the nutritive region. In addition to being the first specimen of Deinosuchus from northeastern North America described in detail, the tooth emphasizes the biological extremes of attaining large body size in Deinosuchus and may add support to the hypothesis that the ontogeny of gigantic crocodyliforms was characterized by extended periods of juvenile growth.