Galectin-1 (Gal1) plays a regulatory role in the immune system. We have recently validated that Gal1 serum (sGal1) levels are increased in rheumatoid arthritis (RA) patients compared to healthy donors (HDs); however, there is no information on Gal1 in spondyloarthritis (SpA). Objective: To compare Gal1 levels in patients with SpA versus RA as a diagnostic biomarker. Methods: We studied sGal1 levels in HD (n = 52), SpA (n = 80) and RA patients (n = 64) who were randomly divided into discovery and validation sets. Synovial fluid (SF) from osteoarthritis (OA) (n = 28), peripheral SpA (n = 28) and RA (n = 28) were studied. In SpA patients, we analyzed the association between clinical parameters and sGal1 levels. Results: sGal1 levels were significantly lower in patients with SpA with respect to RA and similar to those of the HD. A cut-off of 20.50 ng/mL (sGal1) allowed one to differentiate RA patients from SpA and HD (Odd Ratio (OR) 8.23 and 12.64, respectively). Gal1 SF levels in SpA were slightly lower than OA patients and significantly lower than RA patients. No correlation was observed between sGal1 levels and clinical parameters in SpA patients. Conclusion: Gal1 could act as a diagnostic biomarker of RA and would allow one to distinguish SpA and RA patients.