 Mn2O3 microcubes with three-dimensional (3D) micromesoporous network structure were synthesized by thermal decomposition of the as-prepared MnCO3 precursor. The structure, morphology, and textural property of the as-synthesized Mn2O3 microcubes were characterized. The electrochemical property and electrochemical sensing performance toward nitrite detection were investigated. Owing to the unique 3D micromesoporous network structure, the Mn2O3 microcubes could provide electrochemical sensing of nitrite with high sensitivity, selectivity, and reliability.