A hilly-terrain pipeline consists of horizontal, upward inclined and downward inclined sections. The lack of understanding of how three-phase slug flow characteristics change in hilly-terrain pipelines may lead to inaccurate modeling of the phenomenon and thus poor pipeline and downstream facility designs. Although several slug tracking models are available, their performance has not been thoroughly tested against gas-oil-water data in hilly-terrain pipelines due to the scarcity of such data. Three-phase gas-oil-water slug flow evolution in hilly-terrain pipelines was studied experimentally. The constructed experimental facility was a 69 m long, 50.8 mm ID outdoor facility with ± 5° inclination angle for a valley configuration. Threephase slug flow developments in the hilly-terrain section were observed and analyzed with the measured pressure drop, average liquid holdup, phase distributions and slug characteristics. This study improves the current understanding of gas-oil-water flow behavior in hilly-terrain pipelines and the effect of water cut on slug flow characteristics. This understanding will improve the existing slug tracking models or can be used to develop new models when, necessary, for the proper design and safe operation of three-phase pipeline systems. Seven three-phase slug flow patterns were identified based on oil-water mixture in the upstream horizontal section of the hilly-terrain unit. These flow patterns were analyzed and compared with slug dissipation in the downhill section of the hilly-terrain unit. When these flow patterns were compared with two-phase slug dissipation behavior, no water cut effect was observed. For moderate and high flow rates, slugs with different oil-water mixing status had differences in slug frequencies and lengths. However, the evolution of liquid slug length distributions for 20% and 80% water cuts in the upstream horizontal section and upward inclined section did not show any significant dependence on water cut.