Abstract Background To evaluate the association between weight misperception and psychological symptoms in the Determinants of young Adults Social well-being and Health (DASH) longitudinal study. Methods A longitudinal sample of 3227 adolescents, in 49 secondary schools in London, aged 11–16 years participated in 2002/2003 and were followed up in 2005/2006. A sub-sample (N = 595) was followed up again at ages 21–23 years in 2012/2013. An index of weight misperception was derived from weight perception and measured weight. Psychological well- being was measured using the Strengths and Difficulties Questionnaire at 11–16 years and the General Health Questionnaire at 21–23 years. Associations with weight misperception was assessed using regression models, adjusted for socio-economic and lifestyle factors. Results White British males and females were more likely than ethnic minority peers to report accurate perceptions of measured weight. At 11-13y, 46% females and 38% males did not have an accurate perception of their measured weight. The comparable figures at 14-16y were 42 and 40%. Compared with male adolescents, more females perceived themselves as overweight or were unsure of their weight but measured normal weight, and this was more pronounced among Indians, Pakistanis and Bangladeshis. At 14-16y, more males perceived themselves as underweight but measured normal weight, and this was more pronounced among Indians. Compared with those who had an accurate perception of their normal weight, a higher likelihood of probable clinically-relevant psychological symptoms was observed among those who measured normal weight but perceived themselves to be underweight (females Odds Ratio (OR) = 1.87 95% CI 1.03–3.40; males OR = 2.34 95% CI 1.47–3.71), overweight (females only OR = 2.06 95% CI 1.10–3.87), or unsure of their weight (males only OR = 1.61 95% CI 1.04–2.49). Among females, the association was driven by internalising rather than externalising symptoms. An accurate perception of overweight was associated with higher psychological symptoms in adolescence and early 20s. Ethnic specific effects were not evident. Conclusion Weight misperception may be an important determinant of psychological symptoms in young people, with an accurate perception of normal weight status being protective. Culturally targeted interventions should be considered to promote healthy perceptions of body image.