Juvenile idiopathic arthritis can affect the temporomandibular joint (TMJ) can cause growth disturbances of the lower jaw (mandible). The collagen-induced arthritis (CIA) juvenile rat model may be an appropriate model for studying how juvenile arthritis affects this joint during growth. However, studies using this animal model to investigate TMJ arthritis are limited. To validate an animal model for studying TMJ arthritis in growing rats, our study aimed to investigate the changes in mandibular growth and expression of proteins and cytokines in the mandibular condyle of CIA juvenile rat TMJs. A total of 27 male Wistar rats (3 weeks old) were scanned with microcomputed tomography (MicroCT) and divided into three groups (n = 9); CIA was induced in each TMJ in the CIA group, the Saline group received saline injections (sham injections) into their TMJs, and the Healthy group remained untreated (no TMJ injections) as negative controls. After 4 weeks, our results show that mandibular growth was significantly reduced in the CIA group compared with the Saline group (p < 0.01). There was no difference in mandibular growth between the two control groups (Saline and Healthy). Inflamed synovial tissue, cartilage invaginations, and lipid accumulation were observed in the CIA TMJs. Toluidine blue staining revealed decreased proteoglycan production in the CIA cartilage. In addition, immunohistochemistry revealed that type II collagen expression decreased, interleukin-1β expression increased, and matrix metalloproteinase-13 expression increased in the CIA TMJs in comparison with the two control groups (Saline and Healthy). Immunostaining of tumor necrosis factor-α (TNF-α) was quantified and we showed that TNF-α expression was significantly greater in the CIA cartilage compared with both control groups (p < 0.05), and there was no difference in TNF-α expression between the Saline and Healthy groups. This CIA juvenile rat model of TMJ juvenile arthritis shows that CIA reduced mandibular growth and induced degenerative changes in TMJ condylar cartilage. This new information will help to understand the pathogenesis involved in CIA in juvenile rat TMJs for this animal model to be used in research investigating new therapeutics to treat TMJ juvenile arthritis. Impact statement In this study, the effects of collagen-induced arthritis (CIA) on the temporomandibular joint (TMJ) using a juvenile rat model were investigated. Our results showed that local injection of CIA in the TMJ significantly reduced mandibular growth and caused degenerative changes in condylar cartilage. This information helps to validate this animal model for studying the effect of arthritis in TMJs in growing rats. This model has the potential to be used in future studies to evaluate possible therapies for TMJ arthritis.