Objective This study systematically evaluated the effect of intra-articular injection of platelet-rich plasma (PRP) and mesenchymal stem cells (MSC) on knee osteoarthritis (KOA). Methods Randomized controlled trials (RCTs) of PRP combined with MSC in the treatment of KOA were collected from PubMed, Cochrane Library, Web of Science, Wiley online library, CNKI, and Wanfang databases from inception to July 30, 2022. Two researchers read and screened the literature to extract the data, respectively. After conducting a risk-of-bias assessment of included data, RevMan 5.3 software was used for meta-analysis. The Cochrane Handbook risk-of-bias assessment tool was used to evaluate the included literature. Results A total of 9 papers with 480 KOA patients were included in this study. Significant differences in terms of visual analogue scale (VAS) score (MD = −1.10, 95% CI −1.91 to −0.29), P = 0.008) and knee injury and osteoarthritis outcome score (KOOS) (MD = 4.56, 95% CI 0.54–8.58, P = 0.050) were noted between the 2 groups. Subgroup analysis found that MD = 1.66 in KOOS-pain-1 group (95% CI (0.10, 3.22), P = 0.040), which is significant. The MD for KOOS total score and IKDC scores between the two groups was MD = 6.31 (95% CI 2.73–9.88, P = 0.0005) and MD = 3.05 (95% CI −7.09–13.20, P = 0.56), respectively. Conclusion The results of this study provide a theoretical basis for the clinical treatment of KOA with PRP combined with MSC.