This study investigated the concurrent validity of two accelerometer-based physical activity (PA) monitors, the wrist-worn Fitbit Flex, and ActiGraph’s hip-worn GT3X+. Specifically, we examined the relationship, differences, and level of agreement between Fitbit and GT3X+ sedentary behavior (SED) and moderate-to-vigorous PA (MVPA) estimates. Sixty-seven adults (mean age: 47.1 ± 14.1, female: 73.1%) from North Dakota State University wore the Fitbit and GT3X+, and logged any sleep and non-wear time, for seven consecutive days in free-living conditions. GT3X+ estimates were calculated using ActiGraph Freedson, Troiano, and Freedson’s VM3 cut-points. Fitbit estimates were calculated via Fitabase. Only data during waking hours where both PA monitors were worn were analyzed. Fitbit and GTX+ estimates strongly correlated. Fitbit produced similar SED (mean difference = -35.83 minutes (min)/day), but significantly higher MVPA (mean differences = -59.7 – 77.41 min/day) to GT3X+. As the mean volume of MVPA increased, so did differences between Fitbit and GT3X+ estimates.