Background Psoriatic arthritis (PsA) is a seronegative (rheumatoid factor-negative) usually Cyclic Citrullinated Peptide (CCP) antibody negative, immunologically triggered, chronic inflammatory joint disease within the framework of psoriasis. Psoriasis affects 2–4% of the general population. It is a common, chronic, relapsing/remitting, immune-mediated systemic disease characterized by skin lesions including red, scaly patches, papules and plaques, which usually itch. Vitamin D is actually a fat-soluble steroid prohormone that has endocrine, paracrine and autocrine functions. Aim of the Work: To investigate the level of serum vitamin D in patients with psoriatic arthritis and compare it with patients with psoriasis to show if there is a role of vitamin D in development of arthritis in psoriatic patients or not. Patients and Methods: This study is a cross sectional study in which fifty patients with psoriatic arthritis and psoriasis age range 16-50 years selected from those attending the outpatient clinic and inpatients of rheumatology, physical medicine &rehabilitation department, Al-Azhar university hospitals from 2017 to 2018 and divided into two groups: Group (A): (25) patients with psoriatic arthritis were diagnosed according to the criteria for psoriatic arthritis classification “CASPAR criteria". Group (B): (25) patients with skin psoriasis. In this study we measured of 25 hydroxy vitamin D3 using commercial ELISA kits, ESR, CRP, CBC, Blood urea, serum creatinine, Liver enzymes (SGOT & SGPT), Serum uric acid, Fasting blood glucose, RF, urine analysis, ANA, HCV Ab and HIV Ab. Plain x-ray on both hands postero-anterior view, Plain x-ray on lumbosacral spine antero- posterior and lateral views and Plain x-ray on both sacroiliac joints “cone" view. Results: There was female sex predicliction in psoriatic arthritis group. Otherwise, both groups were comparable regarding age, BMI, residence, co-morbid diseases, disease characteristics, and laboratory investigations, except significant increase of WBCs and inflammatory markers (ESR and CRP) in psoriatic arthritis group. Diseases severity was mild in 72%, moderate in 22% and severe in 6.0%; and there was statistically significant increase of moderate and severe disease in PA when compared to psoriasis group. vitamin D was sufficient in 32.0%, insufficient in 34.0% and deficient in 34.0%; and there was no significant difference between psoriasis and PA groups. In addition, vitamin D levels were increased in psoriasis when compared to PA groups. However, the difference was statistically non-significant. No significant relation was found between vitamin D from one side and each of patient characteristics, laboratory investigations (except significant positive correlation with hemoglobin and RBCs), disease characteristics (except significant inverse (negative) correlation with disease duration and disease severity), X-ray findings, and psoriatic arthritis disease activity indices. Conclusion: Vitamin D deficiency/insufficiency was common among psoriasis and psoriatic arthritis patients and the deficiency was significantly and inversely correlated with disease severity (PASI) and disease duration. Thus, is thought to play a role in development (or a consequence) of psoriasis. On the other hand, there was no correlation with PA disease activity and severity indices. Thus, the association seems to be linked to psoriasis proper. Vitamin D supplementation thought to have a potential therapeutic role in treatment of psoriasis.