Purpose: Environmental factors are known to influence physical activity (PA) levels of healthy people, but little work has explored these factors in people with osteoarthritis (OA). The purpose of this study was to explore the influence of outdoor temperature on PA in people with knee OA. Methods: A total of 38 people with knee OA (10 women; mean age 54 y) wore an accelerometer around their waist for 7 consecutive days. Dependent variables from the accelerometers were three PA measures: (1) activity counts/day and time spent at or above moderate levels of PA with (2) a cut-point of 1,041 activity counts/minute (MVPA1041) and (3) a cut-point of 1,952 activity counts/minute (MVPA1952). Independent variables were age, sex, body mass index (BMI), and maximum daily outdoor temperature. Three linear regression analyses were conducted using the three PA dependent variables and independent variables. Results: After controlling for age, sex, and BMI, maximum daily outdoor temperature explained 9% of the variance in activity counts/day (p=0.042), 10% of variance in MVPA1041 (p=0.032), and 14% of variance in MVPA1952 (p=0.016). Participants who engaged in more PA were younger and were exposed to warmer temperatures. Conclusions: Outdoor temperature and age influence the PA levels of people with knee OA. These factors should be considered when designing PA programmes.