Physical inactivity is a common health problem in female college students, and factors such as weight self-stigma and smartphone addiction may negatively impact physical activity in this population. The aim of the current study was to investigate the associations between these variables and identify the moderating effects of smartphone addiction between weight stigma and physical activity. Using a cross-sectional study, information on the level of physical activity in the past week, weight-related self-stigma, and smartphone addiction, as well as demographics, were collected via an online survey from 391 female college students in Taiwan. Participants in two groups of moderate to high and low physical activity were compared using a zero-order bivariate correlation in terms of independent variables. A moderated mediation model using Model 14 in the Hayes’ PROCESS macro with 1000 bootstrapping resamples was applied to assess moderation effects. There were significant differences in terms of weight status, smartphone addiction, and weight stigma between active and inactive groups (p < 0.001). All independent variables except for age were positively correlated (0.14 < r < 0.45). Multivariate regression models indicated that weight status was associated with weight stigma (odds ratio [OR] = 9.13, p < 0.001; 95% CI = 6.90, 11.35). Weight status (OR = 0.47, p = 0.03; 95% CI = 0.23, 0.93), weight stigma (OR = 0.96, p = 0.03; 95% CI = 0.922, 0.997), and smartphone addiction (OR = 0.11, p = 0.003; 95% CI = 0.03, 0.47) were associated with physical activity. The moderating role of smartphone addiction on the association between weight stigma and physical activity was also identified (OR = 1.05, p = 0.049; 95% CI = 1.0001, 1.1004). The moderating effect of smartphone addiction on the association between weight stigma and physical activity suggests that designing interventions to address the negative impacts of both weight stigma and smartphone addiction may help to improve physical activity in female college students.