Whether transition metals can be entrapped inside fullerenes has remained unclear for a long time. Here mass spectrometric proof of entrapment of the group VIII transition-metal platinum (Pt) in fullerenes is first reported. Theoretical calculations on the example of La2PtC90 show that La2Pt@C-2(99915)-C-90 is the most stable isomer. Unlike other reported endohedral metal atoms, the entrapped Pt atom is negatively charged. This work provides valuable clues for the, synthesis of some important missing endohedral metallofullerenes.