Increasing availability of commercially available physical activity (PA) tracking devices, worn by individuals, provides opportunity for objectively measuring PA. However, because of factors such as cost and willingness to wear the device, it is not always feasible to rely on this method. Differences between PA measured directly with a device such as an accelerometer compared to self-reported PA are well-documented; furthermore, gender differences in agreement between the two methods have been highlighted. No studies, however, have directly compared objective PA assessment via Fitbit device with a validated self-report assessment. Our aim was to explore gender differences in level of agreement between self-reported versus objectively measured PA using a Fitbit device. PA was self-reported by a cohort of male and female veterans participating in a randomized clinical trial of Stay Strong, a mobile-Health intervention aimed at increasing PA and computed as “Active Minutes” (the number of moderate plus vigorous PA per week). AMs were also objectively assessed via a Fitbit Charge 2 device worn by individuals for one week. Regression was used to test for gender differences in agreement between self-reported and Fitbit AMs. 355 (90 women, 265 men) veterans with average age of 39.8 years were included in this study. Women similar PA compared to men based on self-reported AMs (p=.15) but PA levels were lower based on Fitbit-tracked AMs (p=<.001). Among women, 42% and 38% achieved the minimum recommendation of 150+ minutes/week based on self-report and Fitbit, respectively (p = 0.65). Among men, 56% and 62% achieved minimum PA levels based on self-report and Fitbit, respectively (p = 0.19). There was no gender difference (p = 0.13) in agreement between self- versus Fitbit-reported AMs. However, the magnitude of Fitbit AMs had a significant effect on agreement between methods (p<.01); specifically, individuals with lower Fitbit AMs tended to over-report AMs while individuals with higher than average Fitbit AMs tended to under-report. For example, adjusted model predictions for individuals at the 25th percentile (79.5 AMs), median (177 AMs), or 75th percentile (343 AMs) of Fitbit-tracked AMs, self-reported AMs that were 185%, 101% and 62% of their Fitbit AMs, respectively. Primary care providers are increasingly eliciting PA level as a “vital sign” for use within clinics. We found no gender differences in agreement between self-report and Fitbit-tracked PA in contrast with earlier reviews citing gender differences. However, individuals, overall, tended to over-report PA compared to Fitbit-tracked PA when Fitbit-tracked PA levels were low and under-report PA when Fitbit-tracked PA levels were high. Our findings suggest that caution is warranted when eliciting self-reported PA levels depending on the magnitude of reported PA.