Background: Knee osteoarthritis (KOA) is a degenerative joint disorder characterized by chronic pain, functional limitation, and low-grade inflammation. While platelet-rich plasma (PRP) has gained traction as a biologic therapy, the relationship between short-term clinical outcomes and systemic inflammatory biomarker dynamics remains poorly understood in routine clinical settings. Methods: This prospective observational study evaluated 40 patients with grade 2–3 KOA receiving a single ultrasound-guided intra-articular PRP injection. Clinical outcomes via the Visual Analog Scale (VAS), the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), and the Short Form-36 (SF-36), alongside serum inflammatory markers (IL-1β, IL-8, IL-18, TNF-α, erythrocyte sedimentation rate, and C-reactive protein), were assessed at baseline and 4 weeks post-injection using paired tests and multivariable regression. Results: At 4 weeks, clinical scores improved significantly (all p < 0.001). Circulating IL-8 and TNF-α levels decreased significantly (p = 0.009 and p = 0.014, respectively), whereas IL-1β and IL-18 variations were non-significant. Baseline cytokines did not predict clinical outcomes, but a significant association emerged between ΔIL-18 and ΔWOMAC (p = 0.032). Conclusions: A single intra-articular PRP injection was associated with short-term clinical improvements and selected reductions in circulating IL-8 and TNF-α, although the clinical significance of these biomarker changes remains uncertain. Given the observational design and short follow-up, these preliminary findings require confirmation in larger controlled trials.