Background: This study examined the social cognition determinants of social distancing behavior during the COVID-19 pandemic in samples from Australia and the US guided by the health action process approach (HAPA). Methods: Participants (Australia: N = 495, 50.1% women; US: N = 701, 48.9% women) completed HAPA social cognition constructs at an initial time-point (T1), and one week later (T2) self-reported their social distancing behavior. Results: Single-indicator structural equation models that excluded and included past behavior exhibited adequate fit with the data. Intention and action control were significant predictors of social distancing behavior in both samples, and intention predicted action and coping planning in the US sample. Self-efficacy and action control were significant predictors of intention in both samples, with attitudes predicting intention in the Australia sample and risk perceptions predicting intention in the US sample. Significant indirect effects of social cognition constructs through intentions were observed. Inclusion of past behavior attenuated model effects. Multigroup analysis revealed no differences in model fit across samples, suggesting that observed variations in the parameter estimates were relatively trivial. Conclusion: Results indicate that social distancing is a function of motivational and volitional processes. This knowledge can be used to inform messaging regarding social distancing during COVID-19 and in future pandemics.