Pipelines are considered to be the most economical method for large-quantity gas transportation. However, the presence of water in natural gas pipelines causes significant safety and economic issues and must be accurately monitored. In this work, we discuss the feasibility of the emerging technology, terahertz spectroscopy, for monitoring water vapor in natural gas pipelines. We also proposed a novel method by which the transmitted terahertz signal is captured at two different absolute pressures; the differential absorption, subtracted by a quadratic term accounting for the water vapor continuum, was found to be linearly proportional to the partial pressure of water in the vapor phase. Our results indicate that using terahertz waves, as a noncontact spectroscopic technique, combined with our proposed pressure-gradient-based methodology, a minimum of 62 ppm of water vapor can be detected with a path length of 14.7 cm at 100 bar, which satisfies industry requirements.