E-cigarette use among youth remains a public health concern. Although extant literature has examined the perceived harms of cigarette use and secondhand smoke, perceptions of harms associated with secondhand e-cigarette aerosol (SHA) are not well understood. Therefore, we used data from the 2020 U.S. National Youth Tobacco Survey (n = 13,292) in which participants indicated whether SHA caused no harm, little harm, some harm, or a lot of harm. We dichotomized SHA harm perceptions as harmless vs harmful. We included sociodemographics (i.e., age, sex, race/ethnicity, sexual orientation, urbanicity), e-cigarette use characteristics, and SHA exposure as covariates and estimated associations between SHA harm perceptions and each covariate using adjusted logistic regression. Most youth perceived SHA as harmful (87.9 %) compared to harmless (12.1 %). Older youth (vs younger youth) had higher odds of perceiving SHA as harmless, whereas male (vs female) youth had 49 % higher odds (95 % CI: 1.29–1.72) of perceiving SHA as harmless. As the number of days of e-cigarette use in the past 30 days increased (vs non-users), odds of perceiving SHA as harmless increased. Youth exposed to SHA (vs no exposure) in the past 30 days had 35 % higher odds of perceiving SHA as harmless (95 % CI: 1.16–1.57). To conclude, youth SHA harm perceptions varied overall and by sociodemographic characteristics, e-cigarette use, and SHA exposure. Educational campaigns to inform youth of the health risks associated with e-cigarettes and SHA are needed to reduce overall nicotine intake and disparities in nicotine exposure.