<p>Quasicrystals are long-range-ordered materials with atypical rotational symmetries, such as 5-fold, 10-fold or 12-fold symmetries, which are incompatible with crystallographic periodicity. Although spin-glass-like freezing phenomena have been observed in quasicrystals, antiferromagnetic order has not. Here we report experimental evidence for antiferromagnetic order in the icosahedral quasicrystal Au₅₆In_28.5Eu_15.5. Its magnetization curve shows a sharp cusp at a Néel temperature of 6.5 K, and both metamagnetic anomaly below and specific heat anomaly at this temperature are consistent with an antiferromagnetic transition. The appearance of magnetic Bragg reflections in the neutron diffraction data below the Néel temperature further confirms the long-range antiferromagnetic order in this icosahedral quasicrystal. Our discovery resolves the long-standing issue of whether antiferromagnetic order is possible in real quasicrystals, inviting further studies particularly on antiferromagnetic icosahedral quasicrystals and quasiperiodic magnetic order, as opposed to periodic magnetic order generally in condensed-matter physics.</p>