IMPORTANCEPsoriasis is a complex systemic disease with skin involvement, somatic comorbidity, and psychiatric illness (PI). Although this view of psoriasis is widely accepted, potential synergies within this triad of symptoms have not been adequately investigated.OBJECTIVES To investigate the independent association of skin psoriasis and somatic comorbidity with the development of PI and to assess whether skin psoriasis and somatic comorbidity act synergistically to produce a risk of PI that is greater than the additive associations.DESIGN, SETTING, AND PARTICIPANTS Participants were enrolled between January 2005 and December 2010, in this retrospective matched case-control study using secondary (ie, administrative), population-based registry data from Swedish patients in routine clinical care. The dates of analysis were March 2017 to December 2019. Participants were patients with skin psoriasis and control participants without psoriasis matched on age, sex, and municipality, who were all free of preexisting PI.