A novel portable open-ended hollow coaxial cable resonator (OE-HCCR) is proposed and demonstrated for direct relative humidity (RH) measurements without the assistance of any humidity-sensitive material. The sensor is fabricated from a homemade hollow coaxial cable platform that provides ease of modification and improved robustness. The microwave cavity resonator is a metal post-coupled open-ended coaxial resonator, which includes two highly-reflective reflectors, i.e., the metal post and the metal plate-backed open end of the coaxial line. The gap distance between the metal plate and the open end is defined by a circular spacer. By simple one-step adjustments of the thickness of the spacer, the RH measurement sensitivity of the OE-HCCR can be greatly enhanced. The device physics is discussed; numerical calculations and experimental investigations are conducted to verify the sensitivity enhancement of the OE-HCCR for RH sensing. Due to the increased sensitivity, this is the first report of an open-ended coaxial line that is able to directly sense gaseous analyte, to the best of our knowledge. The proposed OE-HCCR with the sensitivity enhancement has the potential for highly sensitive chemical sensing combined with functional materials.