Background. This study examines factors associated with physical activity (PA) and sedentary behaviors (SB) in young adults (18-35 years) and compares objective and subjective assessment measures of PA and SB. Methods. 595 young adults (27.7 ± 4.4 years; 25.5 ± 2.6 kg/m2) enrolled in the Study of Novel Approaches to Weight Gain Prevention (SNAP) trial. Hours/day spent in SB (<1.5 METs) and minutes/week spent in bout-related moderate-to-vigorous intensity PA (MVPA; ≥3 METs and ≥10 min) were assessed using self-report and objective measures. Demographic factors associated with SB and MVPA were also explored (i.e., age, gender, BMI, ethnicity, work and relationship status, and number of children). Results. Objective MVPA (263 ± 246 min/wk) was greater than self-report estimates (208 ± 198 min/wk; p < 0.001) and differed by 156 ± 198 min/wk at the individual level (i.e., the absolute difference). Females, overweight participants, African Americans, and those with children participated in the least amount of MVPA. Objective estimates of SB (9.1 ± 1.8 hr/day; 64.5% of wear time) were lower than subjective estimates (10.1 ± 3.5 hr/day; p < 0.001), differing by 2.6 ± 2.5 hr/day for each participant. Conclusion. Young adults interested in weight gain prevention engage in both high levels of MVPA and SB, with participants self-reporting fewer MVPA minutes and more SB compared to objective estimates. This study is registered at ClinicalTrials.gov (NCT01183689).