OBJECTIVE: Patients with chronic musculoskeletal pain (CMP) have difficulty estimating their level of physical activity (PA). Factors associated with this difficulty have yet to be identified; however, identification could allow for increased accuracy in large-scale PA surveys, and enhanced self-management. The purpose of this study was to determine the relationship of interoception and alexithymia with differences between self-reported and objectively measured PA, and investigate factors as they relate to accurately self-reporting PA. METHODS: A cross-sectional survey of 33 patients with CMP and 32 healthy individuals was conducted from July 2018 to June 2019. We measured differences in Moderate to Vigorous Physical Activity (D-MVPA) using an accelerometer and self-report. A heartbeat tracking task (HTT) was used to measure interoception, and alexithymia was measured using the 20-item Toronto Alexithymia Scale (TAS-20). RESULTS: The CMP group (median 43.5 min/day, IQR 20.6-77.6) showed significantly higher D-MVPA values than the control group (median 22.5 min/day, IQR 6.7-34.9) (p < .001). In patients with CMP, D-MVPA positively correlated with TAS-20 (rho = 0.470, p = .006) and correlated negatively with HTT (rho = -0.390, p = .025). CONCLUSION: Inaccurate self-reported measurements of PA in patients with CMP are associated with alexithymia and interoception. This finding suggests that behavioral interventions targeting alexithymia and interoception in CMP patients could lead to improved self-monitoring.