We report a case of mucoid degeneration of the posterior cruciate ligament (PCL). A 37-year-old male had suffered from right knee pain for several years. The active range of motion of his right knee was -30 extension and 100 flexion, with terminal pain. Magnetic resonance imaging (MRI) showed low proton density and T2WI with fat saturation-high signals in the PCL. Arthroscopic resection of the PCL was carried out, with immediate relief of his symptoms. This was a very rare case of mucoid degeneration of the PCL in a young healthy man. If this disease is considered preoperatively, it can be diagnosed based on characteristic MRI findings.