We performed triaxial compressive creep tests to study water weakening of clinopyroxenite in the diffusion creep regime. All tests were carried out on fine-grained samples at confining pressures between 100 and 300 MPa at temperatures of 1321 to 1421 K for water-saturated and 1398 to 1508 K for anhydrous conditions. Samples were prepared by hot-pressing ground powder of Sleaford Bay clinopyroxenite. Water was added to the sample during the run by dehydration of a talc sleeve. Water-saturated aggregates crept ∼30 times faster than aggregates under anhydrous conditions at a temperature of 1400 K and a pressure of 300 MPa. The stress exponent in both cases was ∼1, and the activation energies under water-saturated and anhydrous conditions were 340 ± 30 and 760 ± 20 kJ/mol, respectively. Under water-saturated conditions creep rate increased with increasing water fugacity to the power 1.4 ± 0.2 with an activation volume for creep of 14 ± 6 × 10-6 m3/mol. Comparison of the water weakening in aggregates of clinopyroxene, olivine, and anorthite deformed in the diffusion creep regime indicates that the creep strength of clinopyroxene has a stronger dependence on the water fugacity than do the creep strengths of olivine and anorthite. In water-rich environments, clinopyroxene will be the weakest component in polycrystalline rocks composed of these phases.