Physical activity (PA) and sedentary behavior may contribute to physical function in adults with intellectual disability (ID). This study examined whether objectively measured PA and sedentary behavior levels are associated with physical performance in adults with ID. Fifty-eight adults with ID (29 women and 29 men, age 44 ± 14 years) underwent a measurement of physical performance with the Short Physical Performance Battery (SPPB) and PA and sedentary time using a hip-worn accelerometer (wGT3X-BT; ActiGraph, Pensacola, FL). Moderate PA and age were significantly associated with the SPPB score (r = .39 and .34, respectively; p < .01). A hierarchical-regression model with moderate PA and age as independent variables indicated that moderate PA was a significant predictor of SPPB (p < .001; R 2 = .153), but age was not (p = .123; R 2 change = .036). Overall, moderate PA was significantly associated with the SPPB score, even after accounting for age, in adults with ID.