Messenger RNA (mRNA) is an emerging drug modality for protein replacement therapy. As mRNA efficiently provides protein expression in post-mitotic cells without the risk of insertional mutagenesis, direct delivery of mRNA can be applied, not only as an alternative to gene therapy, but also for various common diseases such as osteoarthritis (OA). In this study, using an mRNA-encoding interleukin-1 receptor antagonist (IL-1Ra), we attempted anti-inflammatory therapy in a rat model of the temporomandibular joint (TMJ) OA, which causes long-lasting joint pain with chronic inflammation. For the intra-articular injection of mRNA, a polyplex nanomicelle, our original polymer-based carrier, was used to offer the advantage of excellent tissue penetration with few immunogenic responses. While the protein expression was transient, a single administration of IL-1Ra mRNA provided sustained pain relief and an inhibitory effect on OA progression for 4 weeks. The mRNA-loaded nanomicelles provided the encoded protein diffusely in the disc and articular cartilage without upregulation of the expression levels of the pro-inflammatory cytokines IL-6 and tumor necrosis factor-α (TNF-α). This proof-of-concept study demonstrates how anti-inflammatory proteins delivered by mRNA delivery using a polyplex nanomicelle could act to alleviate OA, stimulating the development of mRNA therapeutics.