Objective: Synovitis is a prevalent feature in patients with knee osteoarthritis (OA) and meniscal tear and is associated with pain and cartilage damage. Patient-reported swelling is also prevalent in this population. The aim of this study was to investigate the cross-sectional association between patient-reported swelling and effusion-synovitis detected by magnetic resonance imaging (MRI) in patients with OA and meniscal tear. Methods: We used baseline data from a multicenter, randomized controlled trial, Meniscal Tear in Osteoarthritis Research (METEOR). MRI-identified effusion-synovitis, a proxy for effusion and synovitis on noncontrast MRIs, was graded as none/small versus medium/large. Using MRI-identified effusion-synovitis as the gold standard, we assessed the sensitivity, specificity, and positive predictive value of patient self-reported swelling in the previous week (none, intermittent, constant) to detect effusion and synovitis. Results: We analyzed data from 276 patients. Twenty-five percent of patients reported no swelling, 40% had intermittent swelling, and 36% had constant swelling. Fifty-two percent had MRI-identified medium/large–grade effusion-synovitis. As compared with MRI-identified effusion-synovitis, any patient-reported swelling (versus none) had a sensitivity of 84% (95% confidence interval [95% CI] 77–89), a specificity of 34% (95% CI 26–43), and a positive predictive value of 57% (95% CI 54–61). A history of constant swelling (versus none or intermittent) showed a sensitivity of 46% (95% CI 37–54), a specificity of 75% (95% CI 67–82), and a positive predictive value of 66% (95% CI 58–74). Conclusion: We found that the sensitivity and specificity of patient-reported swelling were modest when compared with effusion-synovitis detected by MRI. These data urge caution against using patient-reported swelling as a proxy of inflammation manifesting as effusion-synovitis.