Young adulthood (ages 18-35 years) is considered a transitional period of life where weight gain is common. However, few studies have assessed physical activity (PA) in young adults and the majority have relied on self-report data. PURPOSE: To quantify time spent in sedentary behaviors and moderate-to-vigorous PA (MVPA) in young adults using both subjective and objective measures. METHODS: As part of the baseline assessments for The Study of Novel Approaches to Weight Gain Prevention (SNAP), 595 young adults (age: 27.7±4.4 years; BMI: 25.5±5.6 kg/m2) wore the Sensewear Armband (SWA) for ≥8 hours/day on ≥4 days/week, as an objective measure of MVPA and sedentary time. A modified version of the Paffenbarger Physical Activity Questionnaire (PPAQ; Paffenbarger 1978) and the Sedentary Behavior Questionnaire (SBQ; Rosenberg 2010) were used to subjectively assess MVPA and sedentary time, respectively. Hours/day spent in sedentary behaviors (<1.5 METs) and minutes/day spent in bout-related MVPA (activities ≥3 METs performed for ≥10min) were calculated for subjective and objective measures. RESULTS: Participants wore the SWA for 7.1±0.8 days for 14.1±1.5 hrs/day. Mean time spent in MVPA was greater in SWA (42.0±39.5 min/day) compared to PPAQ (29.7±28.3 min/day; p<0.001). SWA and PPAQ-assessed MVPA differed by 24.6±30.1 min/day for each participant (i.e., the absolute difference). A greater percentage of participants engaged in ≥150 min/wk of MVPA when measured via SWA (63.5%) compared to PPAQ (54.1%; p<0.0001). Mean sedentary time was lower in SWA (9.1±1.8 hr/day; 64.5% of total wear time) compared to SBQ (10.1±3.5 hr/day; p<0.001). SWA and SBQ-assessed sedentary time differed by 2.6±2.5 hr/day for each participant and differed by ≥1hour/day in 67.4% of participants. CONCLUSION: Objective PA data suggest that young adults who are interested in weight gain prevention engage in high levels of bout-related MVPA (∼300 min/week), with nearly two-thirds achieving the national PA recommendation of ≥150 min/wk. Of concern is that this population also spends nearly two-thirds of their day engaging in sedentary behaviors. On average, subjective assessment significantly underestimated MVPA and overestimated sedentary time, compared to objective measures with large discrepancies observed within an individual.