Objective To examine the prospective association between psychological well‐being related measures and severe obesity development in young and middle‐aged UK adults. Design A longitudinal analysis of two cohort studies. Methods We used data from the 1958 National Child Development Study (NCDS) and the 1970 British Cohort Study (BCS) to examine the association between baseline psychological well‐being related measures (depressive symptoms, life satisfaction and self‐efficacy) and severe obesity development (defined as body mass index – BMI ≥35 kg/m 2 ) and residualized BMI change scores at follow‐up. We analysed repeated measures of baseline and follow‐up pairs with 6‐ to 7‐year follow‐up on average ( n = 22,390 and 23,811 observations in NCDS and BCS, respectively) using panel data logistic and linear models controlling for sociodemographic factors. We conducted additional analyses using analytical sample sizes with longer follow‐up (16–17 years). Results Although a range of sociodemographic factors (e.g., being female, non‐married) were associated with increased risk of severe obesity development, we found limited evidence that psychological well‐being related measures were associated with severe obesity development across cohorts and pooled analyses. Depressive symptoms, life satisfaction and self‐efficacy were, however, associated with relatively small changes in continuous BMI change across analyses, and this tended to be limited to participants without obesity (BMI 18.5 to <30 kg/m 2 ) and not those already living with obesity (BMI 30 to <35 kg/m 2 ) at baseline. Conclusions There is limited evidence that psychological well‐being related measures prospectively predict the development of severe obesity. Poorer psychological well‐being is associated with modest changes in body weight in individuals without obesity.