Rock-socket pile foundations have been widely used in engineering practices. It is important to determine the ultimate side shear resistance of the piles socketed into rock for designers. In this study, the results of 145 compression load tests were collected to examine several issues related to the bearing capacity behavior of rock-socket piles. All these load test results were representative since they were carried out worldwide, on different rock types and rock-socket conditions, for a long time period. Using these available load test case history data, the socketed rock type of piles, diameter and embedment depth of socketed piles, uniaxial compressive strength of rock in nature, and the ultimate bearing side resistances were collected. The ratio of ultimate side shear resistance to unconfined compressive strength of the rock was denoted as the ultimate side shear resistance factor of the piles socketed into rocks. Effects of pile diameter, rocked depth, ratio of rock rocketed depth to diameter, and the unconfined compressive strength of the rock on ultimate side shear resistance and the influential factors were evaluated. Empirical relationships between the ultimate side shear resistance factor and the unconfined compressive strength are suggested, and based on the statistical results, the specific design recommendations for the ultimate side shear resistance factor are provided.