Reaction of N,N′-diphenylselenourea, PhNHC(Se)NHPh, with cis-[PtCl2(PPh3)2] and triethylamine in hot methanol gives the first example of a selenourea dianion complex, [Pt{SeC(=NPh)NPh}(PPh3)2] isolated as a yellow solid in high yield. The structure of the complex was determined by X-ray crystallography and found to be similar to the related thiourea dianion complex [Pt{SC(=NPh)NPh}(PPh3)2], with a Pt---Se---C---N four-membered ring. The antitumour activity (P388 murine leukaemia cells) of the selenourea dianion complex was determined, together with a small selection of related platinum(II)-thiourea dianion complexes (prepared by an analogous method) for comparison. The complex [Pt{SC(=NMe)NMe}(PPh3)2] showed high activity [IC50 1.0 μM], with the other complexes showing moderate to low activity.