The basal lineage of monotremes features an extraordinarily complex sex chromosome system which has provided novel insights into the evolution of mammalian sex chromosomes. Recently, sequence information from autosomes, X chromosomes, and XY-shared pseudoautosomal regions has become available. However, no gene has so far been described on any of the Y chromosome-specific regions. We analyzed sequences derived from Y-specific BAC clones to identify genes with potentially male-specific function. Here, we report the identification and characterization of the mediator complex protein gametologs on platypus Y5 (*Crspy*). We also identified the X-chromosomal copy which unexpectedly maps to X1 (*Crspx*). Sequence comparison shows extensive divergence between the X and Y copy, but we found no significant positive selection on either gametolog. Expression analysis shows widespread expression of *Crspx*. *Crspy* is expressed exclusively in males with particularly strong expression in testis and kidney. Reporter gene assays to investigate whether *Crspx*/*y* can act on the recently discovered mouse *Sox9* testis-specific enhancer element did reveal a modest effect together with mouse *Sox9* + *Sf1*, but showed overall no significant upregulation of the reporter gene. This is the first report of a differentiated functional male-specific gene on platypus Y chromosomes, providing new insights into sex chromosome evolution and a candidate gene for male-specific function in monotremes.