Background: This study compared moderate-to-vigorous physical activity (MVPA) and sedentary behavior in U.S.-born and foreign-born adolescents and young adults, and differences in behavior change from adolescence to young adulthood by nativity. Methods: Data on 2039 U.S.-born and 225 foreign-born participants from Project EAT-III (Eating and Activity in Teens and Young Adults) were used to examine MVPA, television/ DVD/video viewing, and computer use. Participants completed surveys at baseline in Minneapolis/St. Paul, MN secondary school classrooms in 1998-1999 (14.9 ± 1.6 y) and follow-up measures online or by mail in 2008-2009 (25.3 ± 1.6 y). Results: At both time points, foreign-born participants reported significantly lower levels of MVPA than their U.S.-born counterparts (P < .05). Foreign-born females at baseline and follow-up and foreign-born males at follow-up reported less television/DVD/video viewing compared with U.S.-born participants (P < .01). All participants experienced a significant decline in MVPA from baseline to follow-up (P < .001). Between-group analyses revealed a significantly greater decline in television/DVDs/video viewing for the foreign-born males compared with U.S.-born males from baseline to follow-up (mean change: foreign-born: -4.8 ± 1.32 hrs/wk, U.S.-born: -0.6 ± 0.6 hrs/wk; P < .01). Conclusions: Differences in activity patterns between foreign-born and U.S.-born youth into young adulthood may contribute to disparities in chronic disease risk. Nativity, along with the social, environmental, and cultural context, should be considered when designing programs to promote MVPA and prevent obesity.