Subchondral insufficiency fracture of the knee (SIFK) is a non-traumatic condition that has been, historically, associated with the elderly. Early diagnosis and management are essential to prevent evolution to subchondral collapse and secondary osteonecrosis, developing prolonged pain and functional losses. This article presents the case of an 83-year-old patient with severe right knee pain with 15 months of evolution, with sudden onset, and no history of trauma or sprain. Upon observation, the patient presented with a limping gait, antalgic posture with the knee in semi-flexion, pain on palpation of the joint medial line, severe pain on passive mobilization, limited joint range of motion, and a positive McMurray test. The X-ray only demonstrated a gonarthrosis grade 1 in the Kellgren and Lawrence classification with medial compartment affection. Due to the exuberant clinical picture with marked functional compromise, as well as clinical radiological dissociation, MRI was requested to rule out SIFK, which was later confirmed. The therapeutic orientation was then adjusted with an indication for non-weight bearing and analgesia, as well as orientation to an orthopedics consultation to request a surgical evaluation. SIFK is difficult to diagnose and may have an unpredictable outcome due to delayed treatment approaches. This clinical case encourages clinicians to consider subchondral fracture in the differential diagnosis of knee pain when an older patient, with subnormal radiographic findings, reports severe knee pain in the absence of overt traumatic injury.