It is known that millimeter waves (MMW) are absorbed by surface tissue such as skin and the cornea. We previously demonstrated that MMW damages the eye by heat accumulation and transfer. To examine the influence of humidity on such damage, we investigated the difference in MMW-induced ocular tissue temperature increase under high (70%) and low (30%) relative humidity. Eyes of pigmented rabbits (Dutch, 11–15-week old) were exposed unilaterally to 40-GHz 200 mW/cm² MMW for 5 or 30 min at the center of the pupillary area with a spot-focus type lens antenna. Infrared thermography was used to measure corneal surface temperature during exposure. Temperature of corneal stroma and lens nucleus were measured using a fluoroptic thermometer during 0–30-min exposure. To visualize the dynamic change of temperature and its distribution in the anterior chamber, saline containing 0.2% microencapsulated thermochromic liquid crystal (MTLC) was injected into the eye anterior chamber. All three temperature measurements were much higher under high than low humidity at 24 °C room temperature and MTLC results showed the same trend during MMW exposure. These findings indicate that humidity affects MMW-induced ocular temperature elevation. We showed that low humidity during MMW exposure decreased heat accumulation and transfer, whereas high humidity increased the thermal effect. The present data demonstrated that controlling the environmental humidity might have an impact on ocular damage in eyes exposed to MMW.