Abstract Background Promoting the adoption of personal hygiene behaviors known to reduce the transmission of COVID-19, such as avoiding touching one’s face with unwashed hands, is important for limiting the spread of infections. Purpose We aimed to test the efficacy of a theory-based intervention to promote the avoidance of touching one’s face with unwashed hands to reduce the spread of COVID-19. Methods We tested effects of an intervention employing imagery, persuasive communication, and planning techniques in two pre-registered studies adopting randomized controlled designs in samples of Australian (N = 254; Study 1) and US (N = 245; Study 2) residents. Participants were randomly assigned to theory-based intervention or education-only conditions (Study 1), or to theory-based intervention, education-only, and no-intervention control conditions (Study 2). The intervention was delivered online and participants completed measures of behavior and theory-based social cognition constructs pre-intervention and one-week postintervention. Results Mixed-model ANOVAs revealed a significant increase in avoidance of touching the face with unwashed hands from pre-intervention to follow-up irrespective of intervention condition in both studies, but no significant condition effects. Exploratory analyses revealed significant effects of the theory-based intervention on behavior at follow-up in individuals with low pre-intervention risk perceptions in Study 2. Conclusions Results indicate high adoption of avoiding touching one’s face with unwashed hands, with behavior increasing over time independent of the intervention. Future research should confirm risk perceptions as a moderator of the effect theory-based interventions on infection-prevention behaviors.