Belled piers are frequently utilized for self-supporting transmission line towers, and the uplift loading is normally the controlling design load. However, belled piers placed in sloping ground have less uplift resistance than the same foundations built on flat ground. This study elucidates the uplift behavior of axially uplift-loaded belled piers in sloping ground. Two full-scale belled piers were constructed in a loess slope having an inclination of 20° with the horizontal, and pullout load testing was conducted for each belled pier. Both the site conditions and the load tests were documented comprehensively. The uplift load versus vertical shaft-head displacement curve of the belled piers in sloping ground exhibited an initial linear region, a curvilinear transition, and a final linear region, which is similar to that of a typical uplift or compression load–displacement curve for level ground. However, the shaft-head displacement at the lower sloping side was larger than that at the upper sloping side. Therefore, the shaft head rotated in the uplift loading direction, and the instantaneous rotation angle increased with the applied load. Unlike the level ground case, the uplift failure of the belled piers in sloping ground initiated from the edge of the bell at the lower sloping side and propagated into the slope surface as the soils deformed and pushed laterally toward the lower sloping side. The failure cracks on the ground surface were not symmetric and developed mainly on the lower sloping side. A comparison of the elastic limit load and the interpreted failure load of the two belled piers revealed that additional embedment of the foundation in sloping ground can achieve an increase in uplift resistance to match the level ground uplift capacity.