A nearly complete fossil soft-shelled turtle (Trionychidae) from the Eocene Washakie Formation of Wyoming is described and identified as Oliveremys uintaensis, comb. nov. Previously known only from a single shell specimen, O. uintaensis can now be diagnosed using both skull and shell characteristics, including callosified but unsculptured xiphiplastra. “Trionyx” tritor “T.” franciscae, “T.” nelsoni and “T.” scutumantiquum are identified as junior synonyms of O. uintaensis. Parsimony and Bayesian analyses place O. uintaensis and all of its junior synonyms along with multiple previously scored fossil trionychid taxa. Reinterpretation of three characters within the matrix, as well as the addition of fossil taxa, resulted in novel relationships. These relationships were not recovered when only extant trionychids were included in a reinterpreted matrix or when fossil taxa were added without reinterpretation. The results indicate that the addition of fossil taxa is sometimes necessary to detect changes caused by other revisions to the character matrix.