Background and Aims Eleven US states and the District of Columbia have legalized the non‐medical use of marijuana. Public marijuana smoking is generally prohibited, although some states have considered exemptions. This study assessed attitudes about public marijuana smoking, perceptions of harm from marijuana second‐hand smoke (SHS) and self‐reported marijuana SHS exposure.

Design Internet panel survey fielded in June–July 2018.

Setting United States.

Participants US adults aged ≥ 18 years (n = 4088).

Measurements Current (past‐30 day) tobacco product use, current marijuana use, opinions about public indoor marijuana smoking, perceptions of harm from marijuana SHS and self‐reported past‐7 day exposure to marijuana SHS in public indoor or outdoor areas were assessed. Weighted prevalence estimates were computed and correlates were assessed using logistic and multinomial regression.

Findings Overall, 27.4% [95% confidence interval (CI) = 25.7, 29.1] of adults reported past‐week marijuana SHS exposure in indoor and/or outdoor public areas; younger adults, blacks, Hispanics, those in the Northeast or West, and current marijuana and/or tobacco users were more commonly exposed (Ps < 0.0001). More than half of adults (52.4%; 95% CI = 50.7, 54.2) regarded marijuana SHS as harmful, and most (81.0%; 95% CI = 79.5, 82.4) opposed public marijuana smoking. Correlates of favoring public marijuana smoking included being male, younger (Ps < 0.01), black or Hispanic, past‐month tobacco and/or marijuana users and perceiving no/low harm from marijuana SHS (Ps < 0.0001).