AbstractRheumatoid arthritis (RA) is an autoimmune disease characterized by joint inflammation and bone erosions. The glycosylated programmed death‐1 (PD‐1) receptor plays an important role in regulating immune responses and maintaining tolerance. In this study, we focus on two features observed in RA: impaired PD‐1 signalling and Galectin‐3 (Gal‐3) upregulation. We hypothesize that Gal‐3 binds PD‐1 and PD‐1 ligands, potentially contributing to impaired PD‐1 signalling. PD‐1 and Gal‐3 levels in RA synovial fluid (SF) and plasma were evaluated by ELISA. PD‐1 and Gal‐3 interaction was examined by Surface Plasmon Resonance and ELISA. PD‐1, PD‐L1 and Gal‐3 expression on mononuclear cells from SF and peripheral blood as well as fibroblast‐like synoviocytes were examined by flow cytometry. Effects of Gal‐3 and PD‐L1 on osteoclast formation was evaluated by tartrate‐resistant acid phosphatase assay. We show that Gal‐3 binds PD‐1 and PD‐L1. Results demonstrated high expression of PD‐1 and Gal‐3 on mononuclear cells, especially from SF. Gal‐3 inhibited PD‐1 signalling when PD‐L1 was present. Furthermore, a role of Gal‐3 in osteoclast formation was observed in vitro, both directly but also through PD‐1:PD‐L1 inhibition. Effects of Gal‐3 on the PD‐1 signalling axis are proposed to be inhibitory, meaning high Gal‐3 levels in the complex synovial microenvironment are not desirable in RA. Preventing Gal‐3's inhibitory role on PD‐1 signalling could, therefore, be a therapeutic target in RA by affecting inflammatory T cell responses and osteoclasts.