Importance: Psoriasis has been shown to be associated with overweight and type 2 diabetes mellitus. The genetic association is unclear. Objective: To examine the association among psoriasis, type 2 diabetes mellitus, and body mass index (BMI) (calculated as weight in kilograms divided by height in meters squared) in twins. Design, Setting, and Participants: This cross-sectional, population-based twin study included 34 781 Danish twins, 20 to 71 years of age. Data from a questionnaire on psoriasis was validated against hospital discharge diagnoses of psoriasis and compared with hospital discharge diagnoses of type 2 diabetes mellitus and self-reported BMI. Data were collected in the spring of 2002. Data were analyzed from January 1 to October 31, 2014. Main Outcomes and Measures: Crude and adjusted odds ratios (ORs) were calculated for psoriasis in relation to type 2 diabetes mellitus, increasing BMI, and obesity in the whole population of twins and in 449 psoriasis-discordant twins. Variance component analysis was used to measure genetic and nongenetic effects on the associations. Results: Among the 34 781 questionnaire respondents, 33 588 with complete data were included in the study (15 443 men [46.0%]; 18 145 women [54.0%]; mean [SD] age, 44.5 [7.6] years). After multivariable adjustment, a significant association was found between psoriasis and type 2 diabetes mellitus (odds ratio [OR], 1.53; 95% CI, 1.03-2.27; P = .04) and between psoriasis and increasing BMI (OR, 1.81; 95% CI, 1.28-2.55; P = .001 in individuals with a BMI>35.0). Among psoriasis-discordant twin pairs, the association between psoriasis and obesity was diluted in monozygotic twins (OR, 1.43; 95% CI, 0.50-4.07; P = .50) relative to dizygotic twins (OR, 2.13; 95% CI, 1.03-4.39; P = .04). Variance decomposition showed that additive genetic factors accounted for 68% (95% CI, 60%-75%) of the variance in the susceptibility to psoriasis, for 73% (95% CI, 58%-83%) of the variance in susceptibility to type 2 diabetes mellitus, and for 74% (95% CI, 72%-76%) of the variance in BMI. The genetic correlation between psoriasis and type 2 diabetes mellitus was 0.13 (-0.06 to 0.31; P = .17); between psoriasis and BMI, 0.12 (0.08 to 0.19; P < .001). The environmental correlation between psoriasis and type 2 diabetes mellitus was 0.10 (-0.71 to 0.17; P = .63); between psoriasis and BMI, -0.05 (-0.14 to 0.04; P = .44). Conclusions and Relevance: This study determines the contribution of genetic and environmental factors to the interaction between obesity, type 2 diabetes mellitus, and psoriasis. Psoriasis, type 2 diabetes mellitus, and obesity are also strongly associated in adults after taking key confounding factors, such as sex, age, and smoking, into account. Results indicate a common genetic etiology for psoriasis and obesity.