Abstract Background Personality reflects relatively stable and pervasive tendencies in feeling, thinking and behaving. While previous studies have found higher extraversion and lower neuroticism to be linked to higher self-reported physical activity levels, larger studies using accelerometer-measured physical activity are lacking. This study investigated the cross-sectional associations of extraversion and neuroticism with both accelerometer-measured and self-reported physical activity and the role of these personality traits in possible discrepancies between these two measures of physical activity among Finnish adults. Methods Two community-dwelling samples were used in this study: a) 47–55-yr-old women (n = 1098) and b) 70–85-yr-old women and men (n = 314). In both samples, extraversion and neuroticism were assessed by the 19-item short form of the Eysenck Personality Inventory. Physical activity was assessed with hip-worn tri-axial accelerometers and self-reported questions. Regression analyses were adjusted by age, BMI and education. Results In the middle-aged women, neuroticism was negatively associated with accelerometer-measured leisure time moderate-to-vigorous physical activity (β = −.07, p = .036) and with self-reported physical activity (β = −.08, p = .021), while extraversion was positively associated with self-reported physical activity (β = .10, p = .005). No associations of extraversion or neuroticism with physical activity were found in the older men and women. Older adults who scored high in neuroticism reported less physical activity than what was measured by accelerometers (β = −.12, p = .039). Extraversion was not associated with discrepancy between self-reported and accelerometer-measured leisure time physical activity in either sample. Conclusions Neuroticism was associated with lower leisure-time physical activity levels and extraversion with higher self-reported physical activity among middle-aged women. Neuroticism and extraversion were unrelated to physical activity among older adults, but older adults with high neuroticism seemed to underreport their physical activity level. The role of personality in the discrepancy between self-reported and device-based physical activity warrants further research.