Abstract Background mRNA-based COVID-19 vaccines have short- and long-term efficacy in healthy individuals, but their efficacy in patients with psoriasis receiving immunomodulatory therapy is less studied. Objectives To investigate long-term immunity after COVID-19 vaccination in patients with psoriasis receiving immunomodulatory therapy. Methods A prospective cohort study including patients (n = 123) with psoriasis receiving methotrexate (MTX) or biologics and controls (n = 226). Only mRNA-based COVID-19 vaccines administered with standard intervals between doses were investigated. Markers of immunity included SARS-CoV-2 spike glycoprotein-specific IgG and IgA, neutralizing capacity, and interferon-γ release from T cells stimulated with peptides of the SARS-CoV-2 spike glycoprotein. Results The proportion of IgG responders was lower 6 months after vaccination in patients receiving anti-tumour necrosis factor (TNF) treatment compared with controls. Anti-TNF treatment was associated with lower IgG levels (β = −0.82, 95% confidence interval −1.38 to −0.25; P = 0.001). The median neutralizing index was lower in the anti-TNF group [50% inhibition (interquartile range [IQR] 37–89)] compared with controls [98% inhibition (IQR 96–99)]; P < 0.001. Cellular responses were numerically lowest in the anti-TNF group. Conclusions Treatment with anti-TNF has an impact on the immunity elicited by mRNA-based COVID-19 vaccination in patients with psoriasis, resulting in a faster waning of humoral and cellular markers of immunity; however, the clinical implications are unknown.