We report on the mechanical properties of CH3NH3PbI3 photovoltaic perovskite measured by nanoindentation. The Young's modulus (E) of the pristine sample is 20.0 +/- 1.5GPa, while the hardness (H) is 1.0 +/- 0.1GPa. Upon extended exposure to water vapor, both quantities decrease dramatically and the sample changes color from silver-black to yellow. Calculations based on density functional theory support this trend in the mechanical response. Chemical treatment of the degraded crystal in methylammonium iodide solution recovers the color of the pristine sample and the values of E and H within 50%. Published by AIP Publishing.