Osgood–Schlatter disease (OSD) is inflammation of the patellar ligament at the tibial tuberosity (apophysitis). It is characterized by a painful bump just below the knee that is worse with activity and better with rest. Episodes of pain typically last a few weeks to months. One or both knees may be affected and flares may recur.

Risk factors include overuse, especially sports which involve frequent running or jumping. The underlying mechanism is repeated tension on the growth plate of the upper tibia. Diagnosis is typically based on the symptoms. A plain X-ray may be either normal or show fragmentation in the attachment area.

Pain typically resolves with time. Applying cold to the affected area, rest, stretching, and strengthening exercises may help. NSAIDs such as ibuprofen may be used. Slightly less stressful activities such as swimming or walking may be recommended. Casting the leg for a period of time may help. After growth slows, typically age 16 in boys and 14 in girls, the pain will no longer occur despite a bump potentially remaining.

About 4% of people are affected at some point in time. Males between the ages of 10 and 15 are most often affected. The condition is named after Robert Bayley Osgood (1873–1956), an American orthopedic surgeon, and Carl B. Schlatter (1864–1934), a Swiss surgeon, who described the condition independently in 1903.