INTRODUCTION: The aim of this study is to assess 10-year trends (2010-2020) in household secondhand smoke exposure in Hong Kong adolescents from inside their own homes and from their neighbors and analyze changes by SES. METHODS: Data from the 2010-2020 School-based Smoking Survey among students (total responses were 228,623) were analyzed in 2023. Weighted prevalence and temporal trends of secondhand smoke exposure were calculated across years. Prevalence differences and prevalence ratios were used to assess SES disparities. Poisson regression models with robust SEs were used for trend analysis. RESULTS: From 2010 to 2020, the prevalence of secondhand smoke exposure from inside students' homes and from their neighbors increased by 26.8% (from 23.5% to 29.8%) and 142.3% (from 16.8% to 40.7%), respectively. Any household secondhand smoke exposure from these sources increased from 33.2% to 58.5%. The prevalence of home secondhand smoke exposure was significantly higher in 2018 and 2020 than in 2010. Neighbor secondhand smoke exposure showed a continuous upward trend throughout the period. Students with lower parental education levels and from poorer families were more exposed, with disparities by parental education widening in 2018. CONCLUSIONS: Household secondhand smoke exposure in Hong Kong adolescents has increased in recent years, with persistent socioeconomic disparities and a widening in 2018. Targeted policies, such as home smoking bans, are needed to protect adolescents from secondhand smoke exposure and reduce socioeconomic disparities.