Aims The study was to examine the effect of Hylan G-F 20 on the progression of posttraumatic osteoarthritis (PTOA) and the expression of the circadian genes neuronal PAS domain protein 2 (NPAS2) and period 2 (Per2). Main methods We used the anterior cruciate ligament transaction and medial menisectomy (ACLT + MMx) model in Wistar rats. The rats were divided into three groups, the sham-operated group, the Hylan G-F 20-treated group, and the saline-treated group. Rats which underwent ACLT + MMx surgery were injected intraarticularly with, respectively, Hylan G-F 20 or saline once a week for 3 consecutive weeks, starting 7 days after surgery. The gross morphology and histopathology of the experimental knee joints were evaluated at the end of week 6. Expression of the NPAS2 and Per2 genes was measured by real-time PCR. Key findings Hylan G-F 20 suppressed the articular cartilage destruction and synovitis compared to the saline-treated group. Compared to the sham-operated group, the Hylan G-F 20-treated group showed significantly upregulated expression of NPAS2 in cartilage (2.53 ± 0.08-fold higher; p < 0.05) and a non-significant increase in Per2 expression (2.35 ± 1.26-fold higher p = 0.28), while the saline-treated group showed significant downregulation of NPAS2 expression and a non-significant decrease in Per2 expression. Significance Our data suggested that early intraarticular injection of Hylan G-F 20 attenuates the progression of PTOA and significantly upregulates NPAS2 expression. These findings provide a new direction for studying associations between the use of a pharmacological agent, the degenerative process, and circadian gene expression.