Introduction Knee osteoarthritis (OA) is the most common articular disease. Different methods are used to alleviate the symptoms of patients with knee OA, including analgesics, physical therapy, exercise prescription, and intra-articular injections (glucocorticoids, hyaluronic acid [HA], etc). New studies have focused on modern therapeutic methods that stimulate cartilage healing process and improve the damage, including the use of platelet-rich plasma (PRP) as a complex of growth factors. Due to the high incidence of OA and its consequences, we decided to study the long-term effect of intraarticular injection of PRP and HA on clinical outcome and quality of life of patients with knee OA. Method This non-placebo-controlled randomized clinical trial involved 160 patients affected by knee OA, grade 1–4 of Kellgren–Lawrence scale. In the PRP group ( n = 87), two intra-articular injections at 4-week interval were applied, and in the HA group ( n = 73), three doses of intra-articular injection at 1-week interval were applied. All patients were prospectively evaluated before and at 12 months after the treatment by Western Ontario and McMaster Universities Arthritis Index (WOMAC) and SF-36 questionnaires. The results were analyzed using SPSS 16.1 software (RCT code: IRCT2014012113442N5). Results At the 12-month follow-up, WOMAC pain score and bodily pain significantly improved in both groups; however, better results were determined in the PRP group compared to the HA group ( P < 0.001). Other WOMAC and SF-36 parameters improved only in the PRP group. More improvement (but not statistically significant) was achieved in patients with grade 2 OA in both the groups. Conclusion This study suggests that PRP injection is more efficacious than HA injection in reducing symptoms and improving quality of life and is a therapeutic option in select patients with knee OA who have not responded to conventional treatment.