A detailed redescription of Santanichthys diasii is presented, based on several new acid-prepared and very well-preserved specimens. S. diasii has a complete Weberian apparatus and is at present the earliest otophysan fish known (early Cretaceous, Albian). In addition, this fish displays at least one synapomorphy of modern characiforms (large and globular lagenar capsules that extend well lateral to the cranium) and we consequently suggest that it is a stem characiform. If this is correct, it represents a significant temporal extension for characiforms. We discuss the phylogenetic implications of its unique combination of features in light of earlier phylogenetic hypotheses.