To determine the diagnostic value of MR signal within the sacroiliac (SI) joint space in spondyloarthritis (SpA). A retrospective analysis of MRIs of SI joints was performed in 363 patients, aged 16-45 years, clinically suspected of sacroiliitis. Intra-articular SI joint MR signals were categorized as normal, high T1 signal, fluid signal, ankylosis or vacuum phenomenon (VP). These MRI findings were correlated with the final diagnosis, according to the ASAS criteria. Sensitivity, specificity, and positive and negative likelihood ratios (LR) and predictive values were calculated. Presence of intra-articular high T1 signal, fluid signal and ankylosis had a specificity of 95.8 %, 95.3 % and 99.5 % for SpA. High T1 signal, fluid signal and ankylosis were present in 38.4 %, 19.2 % and 17.9 % of SpA patients and in 4.2 %, 4.7 % and 0.5 % of patients without SpA, resulting in LR+ of 9.0, 4.1 and 37.9, respectively. VP was present in 13.2 % of SpA patients and in 20.8 % of patients without SpA, resulting in an LR+ of 0.6. Presence of high T1 signal, fluid signal and ankylosis within the SI joint on MRI have high specificity for SpA. High T1 signal is the most sensitive MRI feature within the SI joint for SpA. aEuro cent MRI of the SI joints is typically obtained for diagnosis of spondyloarthritis. aEuro cent The MR signal within the SI joint itself reflects features of spondyloarthritis. aEuro cent Intra-articular high T1 signal, fluid signal and ankylosis are seen in spondyloarthritis. aEuro cent The vacuum phenomenon makes spondyloarthritis less likely.