ABSTRACT Objectives Our aim was to examine Wireless Physical Activity Monitor (WPAM) use and its associations with contextual factors (age, highest education level, social support, mental health) among adults living with HIV engaged in a community-based exercise (CBE) intervention. Design Quantitative longitudinal observational study. Setting Toronto YMCA, Ontario, Canada. Participants Eighty adults living with HIV who initiated the CBE intervention. Interventions Participants received a WPAM to track physical activity during a 25-week CBE intervention involving thrice-weekly exercise, supervised weekly (Phase 1) and a 32-week follow-up involving independent thrice-weekly exercise (Phase 2). Outcome measures Uptake was measured as participants who consented to WPAM use at intervention initiation. Usage was defined as the median proportion of days participants had great than 0 steps out of the total number of days in the study. We measured contextual factors using a baseline demographic questionnaire (age, highest education level), and median scores from the Medical Outcomes Study-Social Support Scale and Patient Health Questionnaire (mental health), where higher scores indicated greater social support and mental health concerns, respectively. We calculated Spearman correlations between WPAM usage and contextual factors defined as weak (ρ≥0.2, moderate (ρ≥0.4), strong (ρ≥0.6), or very strong (ρ≥0.8). Results Seventy-six of 80 participants (95%) consented to WPAM use. In Phase 1, 66% of participants (n=76) used the WPAM at least one day. Median WPAM usage was 50% (25 th , 75 th percentile: 0%, 87%; n=76) of days enrolled in Phase 1 and 23% (0%, 76%; n=64) of days during Phase 2. Correlation coefficients ranged from weak for age (ρ=0.26) and mental health scores (ρ=-0.25) to no correlation (highest education level, social support). Conclusions Most adults living with HIV consented to WPAM use, however, usage declined over time. Future implementation of WPAMs should consider factors to promote sustained usage by adults living with HIV. Trial Registration NCT02794415 STRENGTHS & LIMITATIONS OF THIS STUDY To our knowledge, this is the first quantitative study to measure wireless physical activity monitor (WPAM) use among adults living with HIV engaged in a community-based exercise intervention. The longitudinal study design enabled us to examine changes in WPAM use over time. Utilizing objective measures of physical activity (WPAM) and self-reported (questionnaire) measures of physical activity enabled us to investigate associations of different measurement approaches of physical activity levels among adults living with HIV. Limitations included variable and incomplete participant data across multiple data sources such as, WPAM synchronization, self-reported step count, and completion of weekly exercise questionnaires.