The appropriate fit and comfort while wearing prosthetic sockets are critical factors that contribute to the successful use of upper-limb prostheses. The prevailing metric in current clinical practice for measuring socket fit is self-reported patient discomfort or pain. This can be problematic because of the subjective nature of self-report and possible insensate portions of the residual limb. Objective methods for measuring socket fit would provide prosthetists with more reliable information needed to minimize discomfort and injury due to tissue breakdown associated with localized pressure and shear. The objectives of this study were 1) to develop a method for quantifying upper-limb socket fit condition by measuring the socket-skin interface pressure distribution and 2) to detect any potential correlation between this pressure and patient discomfort in the residual limb. A total of nine upper-limb prosthetic users participated: three in transradial (TR) group, three in transhumeral (TH) group, and three in shoulder disarticulation (SHD) group. Socket interface pressure was measured with thin-film pressure-mapping sensor system, whereas discomfort score was self-recorded using a visual analog scale (VAS). Pressure and discomfort were measured in 68 arm-weight configurations for the subjects with TR or TH amputation and 70 arm-weight configurations for the subjects with a SHD. The accuracy error of the sensors ranged from 5% to 17%, with posttest errors ranging from 7% to 35%. When testing the repeatability of the same arm-weight configurations, pressure was less variable than discomfort in the TR group, slightly less variable than discomfort in the TH group, and more variable than discomfort in the SHD group. Correlations between pressure and discomfort ranged from no correlation to a strong correlation (r = 0.36, p < 0.001) depending on the subject. Discomfort did not correlate with pressure in any of the three subjects with SHD. The results show that more reliable methods are needed for determining socket fit using socket pressure measurement for clinical application.