The title compound, [Pt(C4H10O3PS)(C15H10ClN3)](C24H20B), has a distorted square-planar coordination geometry at the platinum(II) centre, due to the constraints of the tridentate terpyridine ligand. The Pt-II-bound diethylphosphinothioate ligand takes up a conformation to avoid non-bonding contacts with atoms H6 and H6 ".