Purpose Pedometers, which enable self-monitoring of step counts, are effective in facilitating increases in physical activity. Similar devices which provide real-time feedback on sedentary (sitting) behavior are limited. This study aimed to develop and validate a novel device - the SitFIT - which could accurately measure and provide feedback on sedentary behavior and physical activity. Methods The SitFIT is a triaxial accelerometer, developed by PAL Technologies, which is worn in the front trouser pocket. This enables tracking of thigh inclination and therefore differentiation between sitting and upright postures, as well as tracking of step count. It has a display to provide user feedback. To determine the validity of the SitFIT for measuring sedentary behavior and step counts, 21 men, age 30 to 65 yr, with body mass index 26.6 ± 3.9 kg·m -2 wore a SitFIT in a front trouser pocket and an activPAL accelerometer attached to their thigh for up to 7 d. Outputs from the SitFIT were compared with the activPAL, which was assumed to provide criterion standard measurements of sitting and step counts. Results Mean step counts were approximately 4% lower with the SitFIT than activPAL, with correlation between the two methods being very high (r = 0.98) and no obvious bias from the line of equality (regression line, y = 1.0035x + 418.35). Mean sedentary time was approximately 5% higher with the SitFIT than activPAL, correlation between methods was high (r = 0.84), and the equation of the regression line was close to the line of equality (y = 0.8728x + 38.445). Conclusions The SitFIT has excellent validity for measurement of free-living step counts and sedentary time and therefore addresses a clear need for a device that can be used as a tool to provide feedback on sedentary behavior to facilitate behavior change.