Abstract Introduction The growing use of heated tobacco products (HTPs) has raised concerns about secondhand aerosol (SHA) from HTPs, but few studies have been reported on it. This study aimed to investigate the trends in SHA exposure and their socioeconomic inequalities in Japan. Methods The prevalence of SHA exposure from 2017 to 2020 was estimated using longitudinal internet survey data of 5221 participants, aged 20–69 years in 2017 (baseline), with adjustments using inverse probability weighting for “being a participant in an internet survey”. Multivariable modified Poisson regression models were applied to examine the association between socioeconomic status (ie, educational attainment and equivalent income) at baseline and SHA exposure in 2020 with adjustments for sex and age. Results The estimated prevalence of SHA exposure has consistently increased from 4.5% in 2017 to 10.8% in 2020. Lower educational attainment was associated with a higher risk of SHA exposure (p for trend = 0.010). The covariate-adjusted risks of SHA exposure in participants with a low-education and middle-education level were 1.57 and 1.34 times higher, respectively, than in those with a high-education level. However, significant differences in risks of SHA exposure between participants with low-, middle-, and high-income levels were not observed. Meanwhile, participants with a low-education and middle-education level also had 1.87 and 1.61 times higher risks of secondhand smoke (SHS) exposure from combustible cigarettes than those with a high-education level, respectively. Conclusions Our study revealed a rapid increase in SHA exposure and the existence of educational inequalities in both SHA and SHS exposure. Implications Using longitudinal internet cohort survey data, we found that the prevalence of exposure to secondhand aerosol (SHA) from heated tobacco products (HTPs) rapidly increased to 10.8% in 2020 in Japan. Furthermore, people with lower educational attainment were at higher risk of SHA exposure, suggesting that extensive educational interventions may be necessary to inform the public that although emissions from HTPs contain significantly lesser amounts of harmful and potentially harmful constituents and these compounds than cigarette smoke, they are not harmless and still entail risks, and its long-term effects are unknown. Therefore, future extensive monitoring of SHA exposure is needed.