A high-pressure X-ray diffraction study of manganite (γ-MnOOH) was performed using 10.1 GPa at room temperature. The aim of this study was to determine the compression behavior of an oxyhydroxide with an InOOH-related structure. The pressure-volume data were fitted by a third-order Birch-Murnaghan equation of state with the following parameters: V₀ = 270.47(9) ų, K₀ = 91(3) GPa, and K₀' = 7(1). Analysis of axial compressibility showed that the a-and b-axes were more compressible than the c-axis.