We describe a unique reptilian tooth from the Smoky Hill Chalk Member of the Niobrara Chalk in Trego County, Kansas. Its taxonomic placement cannot be ascertained due to its isolated occurrence, and it is possible that the tooth could have come from a mosasauroid in which the juvenile dentition is not known. However, except for its large size, the specimen closely resembles a right maxillary tooth of a dolichosaurid lizard, Coniasaurus crassidens, and is here referred to as cf. Coniasaurus sp. If it indeed belongs to Coniasaurus, it represents 1) the second Coniasaurus specimen from the Smoky Hill Chalk, 2) the first Coniacian record for the genus, and 3) the largest Coniasaurus tooth known to date, one that could have come from a 1.6 m individual.