Intra-articular injections of human mesenchymal stromal cells (MSCs) and platelet-rich plasma (PRP) have been intensively investigated as therapies for knee osteoarthritis (OA) with positive outcomes. In this work we evaluated weather a combination of the treatments (MSCs + PRP) would be beneficial compared to MSCs alone (MSCs) and standard corticosteroid injection (Control group). Forty seven patients (24 males and 23 females; 53.3 ± 10.7 years old) with radiographic symptomatic knee OA (Dejour grades II–IV) were randomized to receive intra-articular injections of MSCs (n = 16), MSCs + PRP (n = 14) or corticosteroid (n=17). MSCs were obtained after mononuclear cells separation from bone marrow aspiration collected from both posterior iliac crests using Sepax automated closed system and expanded in culture until reaching the number of 4 × 10⁷. PRP was obtained by double-centrifugation of whole blood according to a protocol developed in house. After 12 months follow-up, the MSCs and MSCs+PRP groups achieved higher percentages of expected improvement when comparing to the corticosteroid group for the KOOS-symptoms, pain, function and daily living, domains and global score. For the population older or equal to 60 years old the MSCs+PRP group showed significant superiority for the KOOS-ADL domain at 12 months. Cytokines quantification evidenced anti-inflammatory aspects of the treatments. This work evidences the safety and efficacy of intra-articular injection of MSCs for the treatment of early knee OA, with greater improvement with PRP addition particularly to the older population.