Objectives: There is cumulative evidence in the literature supporting a potential role of faecal calprotectin (FCP) as a biomarker for gut inflammation in spondyloarthritis (SpA). However its relevance in undifferentiated SpA (USpA) is still uncertain. The aim of the current study is to assess the diagnostic significance of FCP levels in patients with differentiated and undifferentiated SpA. Material and methods: A total of 52 differentiated SpA, 33 USpA and 50 controls could be included. For all patients, clinical evaluation, routine laboratory investigations, FCP levels, and occult blood in stool were performed. When indicated imaging and/or endoscopies were performed. Results: The differentiated SpA patients were 12 (23.1%) with ankylosing spondylitis, 21 (40.4%) with psoriatic arthritis, 13 (25%) with ulcerative colitis, 5 (9.6%) with Crohn's disease (CD) and one (1.9%) with reactive arthritis. The mean FCP level in 85 patients correlated with CRP and ESR. Within the SpA group ulcerative colitis and Crohn's disease patients had increased FCP levels compared to other SpA subgroups and USpA patients (p < 0.001). The mean FCP levelwas significantly higher in the SpA patients compared to USpA and controls (p < 0.001). Conclusions: Elevated FCP levels may identify patients who are most likely to have SpA already in the unclassified phase of the disease. Further studies in different series of patients are needed to evaluate the potential diagnostic and prognostic roles of FCP in both differentiated and undifferentiated phases of the disease.