What scientific phylum is the platypus in?

Like all mammals, the platypus is in the phylum Chordata.

All animals which have a spine, or backbone, are classified in the phylum Chordata. There are three subphylums in Chordata: Urochordata (tunicates), Cephalachordata (lancelets), and Vertebrata (vertebrates). The platypus is a vertebrate. This phylum also includes all animals which have a hollow nerve cord and a notochord at some stage during their development. In the case of vertebrates, the notochord is present in the embryo, and develops into the vertebral body.