Inflammation-predominant osteoarthritis is an important clinical type of osteoarthritis, with synovitis suggested as its distinct pathophysiology. We investigated whether the synovium's mechanical properties in knees differed by osteoarthritis and other clinical parameters through retrospectively analyzing intra-articular pressure–volume characteristics. We analyzed 60 knees that were administered intra-articular corticosteroids while undergoing pressure monitoring. McMurray's test, pain complaints at end-range knee flexion, Kellgren–Lawrence classification from standing anteroposterior radiographs, and suprapatellar effusion from ultrasound constituted clinical parameters. Pressure–volume profiles—phasic changes in pressure by volume infusion, the volume of Phase 1—the potential volume of the synovial space, the pressure at 45 ml infusion—intra-articular pressure at a standardized volume, and the slope of Phase 2—synovial stiffness were compared with clinical parameters. All graphs were biphasic. Knees with suprapatellar effusion or radiologically definite osteoarthritis (Kellgren–Lawrence grade ≥2), had a lower Phase 1 volume. Knees with definite radiographic osteoarthritis also showed higher pressures at 45 ml and Phase 2 slopes (171.11 ± 94.35 mmHg and 5.08 ± 3.07 mmHg/ml, respectively) than those without (101.88 ± 58.12 mmHg and 2.84 ± 1.27 mmHg/ml, respectively). The Phase 2 slope was higher for knees with positive provocative tests than in those with negative provocative tests, although not statistically significant. The synovium stretched earlier in knees with effusion or radiologically definite osteoarthritis. Intra-articular pressure and synovial stiffness were significantly higher in patients with radiologically definite osteoarthritis. The synovium's mechanical characteristics are altered by osteoarthritis of the knee joint. Intra-articular pressure characteristics could be utilized for synovial evaluation clinically.