BACKGROUND: Evidence suggests an association between atopic eczema (AE) or psoriasis and mental illness; however, the factors associated with mental illness are unclear. OBJECTIVES: To synthesize and evaluate all available evidence on factors associated with depression, anxiety and severe mental illness (SMI) among adults with AE or psoriasis. METHODS: We searched electronic databases, grey literature databases and clinical trial registries from inception to February 2022 for studies of adults with AE or psoriasis. Eligible studies included randomized controlled trials (RCTs), cohort, cross-sectional or case-control studies where effect estimates of factors associated with depression, anxiety or SMI were reported. We did not apply language or geographical restrictions. We assessed risk of bias using the Quality in Prognosis Studies tool. We synthesized results narratively, and if at least two studies were sufficiently homogeneous, we pooled effect estimates in a random effects meta-analysis. RESULTS: We included 21 studies (11 observational, 10 RCTs). No observational studies in AE fulfilled our eligibility criteria. Observational studies in people with psoriasis mostly investigated factors associated with depression or anxiety - one cross-sectional study investigated factors associated with schizophrenia. Pooled effect estimates suggest that female sex and psoriatic arthritis were associated with depression [female sex: odds ratio (OR) 1.62, 95% confidence interval (CI) 1.09-2.40, 95% prediction intervals (PIs) 0.62-4.23, I2 = 24.90%, τ2 = 0.05; psoriatic arthritis: OR 2.26, 95% CI 1.56-3.25, 95% PI 0.21-24.23, I2 = 0.00%, τ2 = 0.00] and anxiety (female sex: OR 2.59, 95% CI 1.32-5.07, 95% PI 0.00-3956.27, I2 = 61.90%, τ2 = 0.22; psoriatic arthritis: OR 1.98, 95% CI 1.33-2.94, I2 = 0.00%, τ2 = 0.00). Moderate/severe psoriasis was associated with anxiety (OR 1.14, 95% CI 1.05-1.25, I2 0.00%, τ2 = 0.00), but not depression. Evidence from RCTs suggested that adults with AE or psoriasis given placebo had higher depression and anxiety scores compared with comparators given targeted treatment (e.g. biologic agents). CONCLUSIONS: Our review highlights limited existing research on factors associated with depression, anxiety and SMI in adults with AE or psoriasis. Observational evidence on factors associated with depression or anxiety in people with psoriasis was conflicting or from single studies, but some identified factors were consistent with those in the general population. Evidence on factors associated with SMIs in people with AE or psoriasis was particularly limited. Evidence from RCTs suggested that AE and psoriasis treated with placebo was associated with higher depression and anxiety scores compared with skin disease treated with targeted therapy; however, follow-up was limited. Therefore, long-term effects on mental health are unclear.