Superparamagnetic iron oxide nanoparticles (SPIONs) are attractive materials that have been widely used in medicine for diagnostic imaging and therapeutic applications. In our study, SPIONs and the corticosteroid dexamethasone acetate (DXM) are co-encapsulated into PLGA microparticles for the aim of locally treating inflammatory conditions such as arthritis. The magnetic properties conferred by the SPIONs could help to maintain the microparticles in the joint with an external magnet. The aim of this study was to investigate the interaction between magnetic microparticles and human synovial fibroblasts in terms of microparticle uptake (FACS, confocal and optical microscopy), internalization mechanism (Prussian Blue staining, TEM, immunofluorescence), cell toxicity (MTT) and tissue reaction after intra-articular injection (histology). The results show that the microparticles have an excellent biocompatibility with synoviocytes and that they are internalized through a phagocytic process, as demonstrated by fluorescence-activated cell sorting and morphological analyses of cells exposed to microparticles. Histological analysis showed that the prepared microparticles did not induce any inflammatory reaction in the joint. This type of carrier could represent a suitable magnetically retainable intra-articular drug delivery system for treating joint diseases such as arthritis or osteoarthritis.