Distinction of hydatidiform moles from nonmolar specimens and their subclassification as complete (complete hydatidiform mole) versus partial hydatidiform mole (PHM) are important for clinical practice and investigational studies to refine ascertainment of risk of persistent gestational trophoblastic disease, which differs among these entities. Immunohistochemical analysis of p57 expression, a paternally imprinted maternally expressed gene on 11p15.5, and molecular genotyping are useful for improving diagnosis. Here, we describe a first trimester abortus with morphologic features consistent with a hydatidiform mole and p57 expression pattern supporting a diagnosis of PHM. Short tandem repeat (STR) genotyping and fluorescent in-situ hybridization analysis showed tetraploidy with 3 paternal and 1 maternal chromosome complements. To our knowledge, this is the first description of a tetraploid PHM confirmed to be triandric by STR analysis, and the first description of p57 immunostaining in a confirmed triandric tetraploid PHM. This case highlights the complex nature of the genetics that can be encountered in molar specimens and illustrates that STR genotyping, in contrast to fluorescent in-situ hybridization or ploidy analysis, offers the advantage of determining the parental origin of chromosome complements for refined diagnosis of hydatidiform moles.