IMPORTANCE Psoriasis is a chronic inflammatory disease. The Mediterranean diet has been shown to reduce chronic inflammation and has a positive effect on the risk of metabolic syndrome and cardiovascular events. Thus, we hypothesized a positive effect on the onset and/or severity of psoriasis. OBJECTIVE To assess the association between a score that reflects the adhesion to a Mediterranean diet (MEDI-LITE) and the onset and/or severity of psoriasis. DESIGN. SETTING, AND PARTICIPANTS The NutriNet-Sante program is an ongoing, observational, web-based questionnaire cohort study launched in France in May 2009. The present study was performed within the framework of the NutriNet-Sante program, with data collected and analyzed between April 2017 and June 2017. Patients with psoriasis were identified via a validated online self-completed questionnaire and then categorized by disease severity: severe psoriasis, nonsevere psoriasis, and psoriasis-free. Data on dietary intake (including alcohol) were gathered during the first 2 years of participation in the cohort to calculate the MEDI-LITE score (ranging from 0 for no adherence to 18 for maximum adherence). Potentially confounding variables (eg, age, sex, physical activity, body mass index, tobacco use, and a history of cardiovascular disease) were also recorded. Analyses used adjusted multinomial logistic regression to estimate the risk of having severe psoriasis or nonsevere psoriasis compared with being psoriasis-free. RESULTS Of the 158 361 total NutriNet-Sante participants, 35 735 (23%) replied to the psoriasis questionnaire. The mean (SD) age of the respondents was 47.5 (14.0) years; 27 220 (76%) of the respondents were women. Of these 35 735 respondents, 3557 (10%) individuals reported having psoriasis. The condition was severe in 878 cases (24.7%), and 299 (8.4%) incident cases were recorded (those arising more than 2 years after participant inclusion in the cohort). After adjustment for confounding factors, a significant inverse relationship was found between the MEDI-LITE score and having severe psoriasis: odds ratio (OR), 0.71; 95% CI, 0.55-0.92 for the MEDI-LITE score's second tertile (score of 8 to 9); and OR, 0.78; 95% CI, 0.59-1.01 for the third tertile (score of 10 to 18). CONCLUSIONS AND RELEVANCE Patients with severe psoriasis displayed low levels of adherence to the Mediterranean diet; this finding supports the hypothesis that the Mediterranean diet may slow the progression of psoriasis. If these findings are confirmed, adherence to a Mediterranean diet should be integrated into the routine management of moderate to severe psoriasis.