The purpose of this study was to identify correlates of changes in physical activity (PA) and sedentary behaviour (SB) among university-based young adults in Bangladesh. Data were from a 1-year prospective study with 2 assessment points (baseline n ​= ​573, 20.7 ​± ​1.35 years, 45% female; retention rate 69%, analytical sample ​= ​395). Participants completed a self-administered written survey on PA, SB, health and lifestyle behaviours, and sociodemographics. Changes in PA were categorised as: negligible (± < 60 ​min/week), > 60 ​min/week decrease, or > 60 ​min/week increase. Changes in SB were categorised as negligible (± < 120 ​min/week), > 120 ​min/week decrease, and > 120 ​min/week increase. Multinomial logistic regression analysis was used to identify the correlates. About quarters (72%) of participants had insufficient PA at both assessment points. Of those who were sufficiently active at Wave 1, 5% became insufficiently active at Wave 2. One quarter of participants (23%) had high SB at Wave 1 and Wave 2. Of those who had low SB at Wave 1, 16% had high SB at Wave 2. Being male [OR ​= ​2.04 (95% CI: 1.06–3.93)], baseline phone time of > 2 h/day [OR ​= ​3.14 (95% CI: 1.04–7.04)] and not participating in organised sports at baseline [OR ​= ​2.56 (95% CI: 1.24–5.29) were associated with a decrease in PA by ​> ​60 ​min/week. Participants who frequently experienced stress at baseline had higher odds of increasing SB by ​> ​120 min/day [OR ​= ​1.83 (95% CI: 1.04–3.23)]. SB is more variable than PA over 1 year in university-based young adults in Bangladesh. Males, those with high phone time, those not engaging with organised sports, and those with frequent stress may change to a more inactive lifestyle.