The elastic properties of jarosite were investigated using synchrotron X-ray diffraction coupled with a multi-anvil apparatus at pressures up to 8.1 GPa. With increasing pressure, the c dimension contracts much more rapidly than a, resulting in a large anisotropy in compression. This behavior is consistent with the layered nature of the jarosite structure, in which the (001) [Fe(O,OH)6]/[SO4] sheets are held together via relatively weak K-O and hydrogen bonds. Fitting of the measured unit-cell parameters to the second-order Birch-Murnaghan equation of state yielded a bulk modulus of 55.7 ± 1.4 GPa and zero-pressure linear compressibilities of 3.2 × 10−3 GPa−1 for the a axis and 13.6 × 10−3 GPa−1 for the c axis. These parameters represent the first experimental determination of the elastic properties of jarosite.