Background: Osteoarthritis (OA) is a painful, chronic condition affecting 40% of those >70 years [1]. Inflammation is an aetiological factor in OA [2]. Blueberries are rich in polyphenols, which are anti-inflammatory [3]. Previously, blueberry supplementation reduced arthritic pain [4]. This study assessed the effect of blueberry supplementation on a) the inflammatory pathophysiology of OA and b) pain and joint function in patients undergoing total knee replacement (TKR). Methods: All procedures accorded with ethical standards and the principles of the World Medical Association’s Declaration of Helsinki. Sixty-one patients scheduled for total knee replacement (TKR) were randomised to receive blueberry powder (c.480 mg anthocyanins/day) (n=32, 69 ± 8 years; BMI 32.1 ± 5.5 kg/m²) or placebo powder (n=29, 69 ± 7 years; BMI 31.7 ± 5.1 kg/m²) for 6 weeks pre- and post-TKR. Serial assessments of pain and joint function were performed using the validated WOMAC questionnaire at baseline, week 3, pre-TKR, 1-week post-TKR, 3 weeks post-TKR and 6 weeks post-TKR. During TKR, synovial fluid, synovium, infrapatellar fat pad (IFP), articular cartilage, and subchondral bone were collected. Gene expression analysis was conducted using Taqman OpenArray targeting 56 genes related to OA pathophysiology. Differential gene expression analysis was performed using limma in R (v 4.3.2) on −ΔCT-transformed qPCR data (blueberry vs. placebo). Synovial fluid samples were profiled using multiplex Luminex assays. Group comparisons were performed using t-tests. Linear mixed-effects models were fit for WOMAC outcomes. Tandem-mass-tagged proteomic analysis was performed on a subset of synovium samples (N=30). Data were analysed using Reactome differential pathway analysis (blueberry vs placebo) via the CAMERA algorithm. Exploratory analysis looked at associations between ssGSEA enrichment scores and post-operative (6–12-week) ΔWOMAC using linear regression. Results: From week 3 onwards, blueberry participants improved by 1.28 WOMAC function points per week, while placebo participants improved more slowly, by 0.69 points less per week (p = 0.041). Blueberry supplementation upregulated adiponectin mRNA in synovium (Log2FC +1.38; p=0.040), IFN-γ in IFP (Log2FC +1.37; p=0.008) and COL13A1 in cartilage (Log2FC 1.58; p=0.039) versus placebo. CX3CL1 mRNA (Log2FC −0.72; p=0.032) and Apolipoprotein E (Log2FC −0.67; p=0.036) were downregulated in bone. Blueberry supplementation reduced insulin concentrations in synovial fluid (mean placebo: 293.2 pg/mL; mean blueberry: 164.5 pg/mL; p = 0.048). Blueberry supplementation significantly altered synovium protein expression in 54 pathways (FDR < 0.05). Pathways related to complement activation (e.g., Initial triggering of complement, Classical antibody-mediated complement activation) and B cell/immune regulation (e.g., CD22-mediated BCR regulation, Antigen activates BCR) were downregulated (log₂FC −0.20 to −0.58, FDR 0.0002–0.036). Pathways including GLUT4 translocation, Membrane trafficking, Cristae formation and Mitochondrial calcium transport were upregulated (log₂FC 0.09–0.23, FDR 0.002–0.047). Several synovium pathways previously modulated by blueberry supplementation were nominally associated with post-operative WOMAC improvements. B cell/immune regulation pathways and complement-related pathways showed positive associations (estimates 5.7–9.2, p 0.009–0.024). These results are exploratory and should be interpreted cautiously due to potential circularity. Conclusions: Blueberry supplementation may ameliorate OA pathology by modulating key molecular pathways involved in joint tissue inflammation and repair.