We present the first water vapor profiles encompassing the upper mesosphere of Mars, 100–120 km, far exceeding the maximum altitudes where remote sensing has been able to observe water to date. Our results are based on solar occultation measurements by Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). The wavelength range observed (~2.7 μm) possesses strong CO2 and H2O absorption lines allowing sensitive temperature and density retrievals. We report the maximum H2O mixing ratio varying from 10 to 50 ppmv at 100–120 km during the global dust storm of MY34 and around southern summer solstice of Martian Years (MY) 34 and 35. During other seasons water remains below 2-3 ppmv. The high values above 100 km establish a bridge between the regular water enrichment observed below (60-100 km) and the atomic hydrogen escaping from the exosphere.