Single crystalline Mn₃O₄ nanowires have been synthesized with tetragonal hausmannite structure using a solvothermal method. The structural and morphological evolution of Mn₃O₄ nanowires have been characterized using powder X-ray diffraction, transmission electron microscopy, and electron resonance spectroscopy. The nanowires were grown uniformly along the (200) direction with a diameter of 5–10 nm range. A relatively broad and intense electron spin resonance (ESR) signal was observed at room temperature, with the g ≈ 2.0. As the synthesis temperature increases from 150 to 250 °C, a decrease in ESR signal intensity and line widths were observed. Mn₃O₄ displayed a positive Curie-Weiss temperature, θ, which decreases with the increase of synthesis temperature.