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Psoriatic arthritis (PsA) is a chronic inflammatory rheumatism and a peripheral subtype of spondyloarthritis. In the absence of specific biological markers, and due to its clinical polymorphism, diagnosis is often difficult and delayed. Anti-citrullinated protein antibodies (ACPAs) constitute an essential diagnostic marker of rheumatoid arthritis (RA) for the rheumatologist. These antibodies are associated with disease severity, including the presence of erosions, extra-articular manifestations, and excess mortality. Their pathophysiological role in psoriatic arthritis has been recently described in several studies, notably, as predictive factors of peripheral clinical severity and of the occurrence of bone erosions in particular. Nevertheless, the prevalence of rheumatoid serology in psoriatic arthritis, although low, is not negligible. The psoriatic arthritic ACPA+ population is under-explored, and this can sometimes lead to a misdiagnosis of RA. In such cases, proper analysis of clinical and radiological symptoms will be of great diagnostic help.