Why Is Only One Knee Swollen?

Noticing swelling in just one knee while the other remains normal can feel concerning. When both knees swell, it often signals a whole-body (systemic) issue like arthritis. However, swelling limited to one knee usually points to a localized problem. Knowing what might cause one swollen knee can help you figure out whether it’s a minor concern or something needing medical care.

1. Injury or Trauma

The most common reason for one swollen knee is injury. Sometimes the cause is obvious, like a fall or sports mishap, but other times, repeated stress or minor strains can lead to swelling without a clear incident. Potential injuries include:

Ligament sprains or tears (like ACL or MCL injuries)
Meniscus tears
Dislocation of the kneecap (patellar dislocation)
Bone bruises or fractures

Typically, swelling appears quickly and may come with pain, stiffness, or a feeling of instability.

2. Bursitis

Bursae are small fluid-filled sacs that cushion the knee joint. If one becomes irritated from pressure, impact, or frequent kneeling, it can cause prepatellar bursitis—often called “housemaid’s knee.” This condition usually affects only one knee, leading to swelling and tenderness at the front of the joint.

3. Infection

An infection inside the knee joint (septic arthritis) is serious and demands urgent care. Symptoms often include rapid swelling, redness, warmth, and intense pain, along with fever or chills. Treatment usually involves draining infected fluid and antibiotics.