The tribological behaviors of silicon nitride (Si3N4) sliding against sintered polycrystalline diamond (PCD) were investigated by varying the relative humidity (RH) in the testing atmosphere. The results indicated that higher RH corresponds to higher wear loss of Si3N4 and the wear loss of PCD almost fell close to zero. Especially in the case of 85% RH, both a maximum wear loss of Si3N4 and a maximum friction coefficient were achieved. In addition, this study revealed insights into the interface chemistry effects on the wear behavior of Si3N4 under humidity. When water molecules were introduced into the testing atmosphere, the hydrolysis reaction occurred on the Si3N4 surface with the formation of the Si-O-Si bond across the sliding interface. And then, the hydration reaction dominated the process, during which Si-OH was formed through the bond fracture of the Si-O-Si. The X-ray photoelectron spectroscopy results showed that the ratios of Si-OH/Si-O and Si-N/Si-OH+Si-O bonds increased as the relative RH levels increased. As a consequence, the wear loss of Si3N4 significantly increased. Thus, due to the hydrolysis and hydration reactions, the tribological behaviors of Si3N4 against sintered polycrystalline diamond can be essentially controlled via varying RH levels.