Exposure to secondhand smoke (SHS) is a health risk to non-smokers. Indoor particulate matter (PM2.5) is associated with SHS exposure and is used as a proxy measure. However, PM2.5 is non-specific and influenced by a number of environmental factors, which are subject to geographical variation. The nature of association between SHS exposure and indoor PM2.5-studied primarily in high-income countries (HICs) context-may not be globally applicable. We set out to explore this association in a low/middle-income country setting, Dhaka, Bangladesh. A cross-sectional study was conducted among households with at least one resident smoker. We inquired whether smoking was permitted inside the home (smoking-permitted homes, SPH) or not (smoke-free homes, SFH), and measured indoor PM2.5 concentrations using a low-cost instrument (Dylos DC1700) for at least 22 hours. We describe and compare SPH and SFH and use multiple linear regression to evaluate which variables are associated with PM2.5 level among all households. We surveyed 1746 households between April and August 2018; 967 (55%) were SPH and 779 (45%) were SFH. The difference between PM2.5 values for SFH (median 27 µg/m3, IQR 25) and SPH (median 32 µg/m3, IQR 31) was 5 µg/m3 (p<0.001). Lead participant's education level, being a non-smoker, having outdoor space and smoke-free rule at home and not using kerosene oil for cooking were significantly associated with lower PM2.5. We found a small but significant difference between PM2.5 concentrations in SPH compared with SFH in Dhaka, Bangladesh-a value much lower than observed in HICs. © Author(s) (or their employer(s)) 2022. No commercial re-use. See rights and permissions. Published by BMJ.