Background/Objectives: The prevalence of inflammatory bowel disease (IBD) in spondyloarthritis (SpA) patients is unknown. Our objective was to assess the prevalence of undiagnosed IBD in SpA patients, including those with axial spondylarthritis (axSpA) or psoriatic arthritis (PsA). Additionally, we examined fecal calprotectin (FC) levels in relation to the accuracy of IBD diagnosis. Methods: EISER was a cross-sectional, multicenter, observational, rheumatologist-gastroenterologist collaborative study. Patients with SpA naive to biologics were recruited. Demographic and clinical characteristics, disease activity, and treatment information were collected. Patients with FC >/= 80 microg/g or IBD-related symptoms underwent a colonoscopy or video capsule endoscopy. Receiver operating characteristic analysis assessed the predictive value of FC for IBD diagnosis. Results: Of the 570 patients recruited, 494 were evaluable for the main outcome, 248 (50.2%) had axSpA, and 246 (49.8%) had PsA. Overall, 28/494 patients were diagnosed with IBD (5.7%, 95%CI 3.6-7.7). Sorted by clinical entity, 22 (8.9%, 95%CI 5.3-12.4) axSpA and 6 (2.4%, 95%CI 0.5-4.4) PsA patients had a diagnosis of IBD: 24 (86%, 95%CI 79.4-92.6) had ileal/ileocolonic Crohn's disease (CD), 3 (11%, 95%CI 5.1-16.9) unclassified IBD, and 1 (3.5%, 95%CI 0.0-6.9) ulcerative colitis. The ROC curve for FC and IBD diagnosis (AUC: 0.870, p < 0.001, 95%CI 83.7-89.8) showed that an FC >/= 147 microg/g had a positive predictive value of 17.4% (95%CI 14.5-20.8) Conclusions: In SpA, the prevalence of undiagnosed IBD was 5.7%, higher in axSpA (8.9%) than in PsA (2.4%) patients, with CD being the most common. SpA patients with FC levels < 147 microg/g had a very low probability of IBD.