Concave octahedral Mn₃O₄ microcrystals were synthesized. The magnetic properties of the obtained concave octahedral Mn₃O₄ microcrystals were investigated from 5 K to 300 K. The blocking temperature was 41.1 K. The Curie–Weiss temperature was 43.5 K, which was higher than that of bulk single-crystal Mn₃O₄ (42 K).