Although psoriasis has been associated with obesity, there are few prospective studies with objective measures. We prospectively examined the effect of body mass index, waist circumference, waist-hip ratio, and 10-year weight change on the risk of developing psoriasis among 33,734 people in the population-based Nord-Trøndelag Health Study (i.e., HUNT), Norway. During follow-up, 369 incident psoriasis cases occurred. Relative risk (RR) of psoriasis was estimated by Cox regression. One standard deviation higher body mass index, waist circumference, and waist-hip ratio gave RRs of 1.22 (95% confidence interval [CI] = 1.11-1.34), 1.26 (95% CI = 1.15-1.39), and 1.18 (95% CI = 1.07-1.31), respectively. Compared with normal weight participants, obese people had an RR of 1.87 (95% CI = 1.38-2.52), whereas comparing the fourth with the first quartile of waist circumference gave an RR of 1.95 (95% CI = 1.46-2.61). One standard deviation higher weight change gave an RR of 1.20 (95% CI = 1.07-1.35), and people who increased their body weight by 10 kg or more had an RR of 1.72 (95% CI = 1.15-2.58) compared with being weight stable. In conclusion, obesity and high abdominal fat mass doubles the risk of psoriasis, and long-term weight gain substantially increases psoriasis risk. Preventing weight gain and promoting maintenance of a normal body weight could reduce incidence of psoriasis.