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.