Osteoarthritis (OA) is a common cause of lameness, pain, and dysfunction in dogs. To determine the levels of interleukin (IL) (IL-1β, IL-6, IL-10), tumor necrosis factor (TNF)-α, and prostaglandin E₂ (PG-E₂) in the synovial fluid after intra-articular injection of an autologous platelet-rich plasma concentrate to treat dogs with osteoarthritis. Twenty dogs with osteoarthritis (OA) were used as a materials. Fourteen dogs were used as a platelet-rich plasma (PRP) treatment group and 6 dogs were used as a control (0.9 % NaCl). PRP was obtained by the double centrifuge method. Affected joints were examined by radiography and ultrasonography. Lameness and pain severity were evaluated by attending clinicians. Samples were collected under sterile conditions at pre-treatment, days 1, 3, 5, 7, 15, and weeks 4, 8, and 12. The marker levels were determined by an enzyme-linked immunosorbent assay. No adverse effects of the injection of platelet concentrate or saline solution were observed. No significant differences were found in the Hudson Visual Analog Scale and Canine Brief Pain Inventory scores between weeks 0, 4, and 12 in the control dogs. No significant changes were observed in IL-1β, IL-6, IL-10, TNF-α, and PG-E₂ levels (P>0.05) in synovial fluid from the PRP treatment group compared to synovial fluid from the control group, although fluctuations in parameter levels were observed in both groups. Despite some variable results in inflammatory parameters, clinical improvement was recorded in the PRP-treated group. PRP injection could be an effective and safe method for treatment of dogs with osteoarthritis.