Background/Aim: Fine particulate matter (PM2.5) is associated to high blood pressure (BP) and cardiovascular disease (CVD); however, most studies have used outdoor as proxy of indoor PM2.5 and office-based or one-point BP measurements not reflecting physiological BP behaviour. We determined the relationship between indoor PM2.5 and 24-hour ambulatory blood pressure (ABP). Methods: Cross-sectional study conducted in adults (≥ 40 years, non-smokers, without CVD) from Bucaramanga, Colombia. We systematically sampled houses within a 1.5 km radius around two PM10/2.5 area monitors of the local air quality network, searching for participants. Indoor PM2.5 was measured in each participant's house with gravimetric sampling pumps (Casella Cel–TUFF; 3L/min flow) during 1-2 consecutive days (≥ 8 hours/monitoring). Measurement of ABP (Spacelab, 90217A) was performed concurrently to indoor monitoring (≥ 60% effective BP measurements were required for inclusion). We collected sociodemographic, anthropometric and physical activity data (GPAQ), and measured fasting blood glucose and lipids. Piecewise regression was used to evaluate the relationship between PM2.5 and ABP adjusting for age, sex, adiposity, physical activity, outdoor PM2.5, and house-to-area monitor distance. Results: We evaluated 395 participants (mean age: 57.6 years; 23% men) and houses (mean indoor PM2.5: 23.7 µg/m3 [SD=14.1 µg/m3]). There were no univariate differences of age, sex, time at home, adiposity, physical activity, ABP or laboratory data by quintiles of indoor PM2.5; however, antihypertensive treatment was more prevalent at lower levels of PM2.5. In multivariable analysis, indoor PM2.5 was positively associated with 24-hour mean systolic BP, but no other ABP indexes, after a threshold of 36.0 µg/m3 in the whole sample (2.3 mmHg per 5 µg/m3 [95%CI: 0.4, 4.1]) and in participants without antihypertensive treatment (2.7 mmHg per 5 µg/m3 [95%CI: 0.7, 4.7]). Conclusions: Indoor PM2.5 is non-linearly related to ambulatory BP in adults, regardless of antihypertensive treatment, with a threshold below Colombia's current normative outdoor PM2.5 concentration.