We report a case of a 63-year-old male with known adenocarcinoma of the lung who presented with knee pain which was initially diagnosed as mucoid degeneration of the anterior cruciate ligament (ACL) on magnetic resonance imaging (MRI). Due to persistent knee pain an interval MRI was performed, followed by image guided biopsy which showed metastatic adenocarcinoma of the lung infiltrating the ACL. This is the first reported case in the literature of an intra-articular metastasis mimicking mucoid degeneration of the ACL.