A swollen knee can be alarming, especially when it affects only one leg. You might wake up one morning and notice that one knee looks noticeably larger than the other, feels stiff, or is uncomfortable to bend. While it’s tempting to ignore the problem or hope it resolves on its own, understanding the possible causes can help you decide whether simple home care is enough or if it’s time to seek medical attention.

The good news is that a swollen knee doesn’t always indicate a serious condition. In many cases, it results from a minor injury, overuse, or temporary inflammation. However, persistent or severe swelling can sometimes point to an underlying medical issue that deserves evaluation by a healthcare professional.

Let’s explore the most common reasons why one knee may become swollen while the other remains perfectly normal.

One of the most frequent causes is a minor injury. Even if you don’t remember twisting your knee or bumping it against something, small injuries can lead to inflammation inside the joint. Activities such as gardening, climbing stairs repeatedly, running, or playing sports may strain the knee enough to cause swelling several hours later.

Ligament sprains are another common possibility. The knee contains several important ligaments that stabilize the joint. A sudden twist, awkward landing, or change in direction can stretch or partially tear these ligaments. Swelling often develops within hours and may be accompanied by pain or a feeling that the knee is unstable.

Meniscus injuries are also relatively common. The meniscus is a piece of cartilage that cushions the knee joint. It can become torn during sports or even while performing everyday activities, particularly in older adults. Besides swelling, a torn meniscus may cause clicking, locking, or difficulty fully straightening the knee.

Inflammation of the bursae, known as bursitis, is another potential explanation. Small fluid-filled sacs called bursae help reduce friction around the knee. Repeated kneeling, prolonged pressure, or irritation can cause one of these sacs to become inflamed, resulting in localized swelling near the front or side of the knee.

Tendon irritation can produce similar symptoms. The tendons connecting muscles to bones sometimes become inflamed after repetitive activities such as running, jumping, or climbing. This condition often develops gradually rather than suddenly and may worsen with continued activity.

Arthritis is another possibility, even if only one knee seems affected initially. Osteoarthritis, the most common form, occurs as joint cartilage gradually wears down over time. Early in the disease, one knee may experience more stress than the other, leading to swelling, stiffness, and discomfort after activity.

Inflammatory forms of arthritis, such as rheumatoid arthritis or psoriatic arthritis, may also involve knee swelling. Although these conditions often affect multiple joints, symptoms don’t always appear equally on both sides of the body. Early evaluation can help identify these conditions before additional joints become involved.

Another possible cause is excess fluid inside the knee joint, often referred to as “water on the knee.” This isn’t a disease itself but rather a sign that the joint is responding to irritation, injury, or inflammation. The accumulated fluid may make the knee feel tight, heavy, or difficult to bend completely.