From high-velocity friction experiments on gabbro rock at a rate of 85 mm/s, we found that a fault with a thin gouge layer experienced slip weakening with a drop of about 80% in the friction coefficient. At room humidity conditions, the fault subsequently recovered its complete strength, over an interval of several minutes after the slip terminated. The strength recovery did not occur under dry N2 gas-saturated condition. This result can be interpreted as the interaction of moisture with gouge particles: moisture-drained weakening due to frictional heating and moisture-absorbed strengthening due to cooling. This new mechanism can cause dynamic weakening of faults, which controls earthquake rupture propagation.