Reaction of the novel thiopropyl-closo-1,2-carborane ligand bearing a pendant glycerol group HS(CH₂)₃CB₁₀H₁₀CCH₂OCH(CH₂OH)₂ (L) with the labile platinum(II) precursor [Pt(MeCN)(terpy)](OTf)₂ (terpy = 2,2′:6′,2″-terpyridine; OTf = trifluoromethanesulfonate) affords the highly water-soluble platinum(II) complex [PtL(terpy)]OTf, the first example of a metal–carborane complex functionalised with a water-solubilising glycerol group.