High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang, and provide an efficient means of producing and studying antimatter. Antimatter nuclei with B<-1 have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1 000 with each additional antinucleon. The antimatter helium-4 nucleus (4He) has not been observed previously. Here we report the observation of 4He, the heaviest observed antinucleus to date. In total, 18 4He counts were detected at the STAR[1] experiment at the Relativistic Heavy Ion Collider in 109 recorded gold-on-gold collisions at centre-of-mass energies of 200 and 62 GeV per nucleon-nucleon pair. The Time Of Flight (TOF) detector, to which Chinese institutions has made major contributions, and the High Level Trigger (HLT) system, in which IMP has deeply involved, are key to this observation. High-energy nuclear collisions create an energy density similar to that of the Universe microseconds after the Big Bang, and provide an efficient means of producing and studying antimatter. Antimatter nuclei with B <-1 have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by a factor of about 1 000 with each additional antinucleon. The antimatter helium-4 nucleus (4He) has not been observed previously. Here we report the observation of 4He, the heaviest observed antinucleus to date. In total, 18 4He counts were detected at the STAR [1] experiment at the Relativistic Heavy Ion Collider in 109 recorded gold-on-gold collisions at center-of-mass energies of 200 and 62 GeV per nucleon -nucleon pair. The Time Of Flight (TOF) detector, to which Chinese institutions have made major contributions, and the High Level Trigger (HLT) system, in which IMP has deeply involved, are key to this observa tion.