At the origin of the Universe, an asymmetry between the amount of created matter and antimatter led to the matter-dominated Universe as we know it today. The origins of this asymmetry remain unknown so far. High-energy nuclear collisions create conditions similar to the Universe microseconds after the Big Bang, with comparable amounts of matter and antimatter^1,2,3,4,5,6. Much of the created antimatter escapes the rapidly expanding fireball without annihilating, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and to study their properties^{7,8,9,10,11,12,13,14}, hoping to shed some light on the existing questions on the asymmetry between matter and antimatter. Here we report the observation of the antimatter hypernucleus \({}_{\bar{\Lambda }}{}^{4}\bar{{\rm{H}}}\), composed of a \(\bar{\Lambda }\), an antiproton and two antineutrons. The discovery was made through its two-body decay after production in ultrarelativistic heavy-ion collisions by the STAR experiment at the Relativistic Heavy Ion Collider^15,16. In total, 15.6 candidate \({}_{\bar{\Lambda }}{}^{4}\bar{{\rm{H}}}\) antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernuclei \({}_{\bar{\Lambda }}{}^{3}\bar{{\rm{H}}}\) and \({}_{\bar{\Lambda }}{}^{4}\bar{{\rm{H}}}\) are measured and compared with the lifetimes of their corresponding hypernuclei, testing the symmetry between matter and antimatter. Various production yield ratios among (anti)hypernuclei (hypernuclei and/or antihypernuclei) and (anti)nuclei (nuclei and/or antinuclei) are also measured and compared with theoretical model predictions, shedding light on their production mechanisms.