A swollen right ankle, without swelling in the left, usually points to something local: an injury, an infection, a blood clot, or inflammation in a specific joint or tendon. When only one ankle swells, the cause is almost always different from the conditions that make both ankles puffy (which tend to involve the heart, kidneys, or liver). The side that’s affected doesn’t matter medically, so everything here applies whether it’s your right or left ankle.
Sprains and Other Injuries
The most common reason for sudden ankle swelling is a sprain. This happens when the ligaments that hold the joint together stretch or tear, triggering pain, swelling, and instability. You don’t need a dramatic fall to sprain an ankle. Stepping off a curb at an odd angle or rolling your foot on an uneven surface is enough.
Swelling from a sprain typically peaks in the first 24 to 48 hours. A mild sprain (where the ligament stretches but doesn’t tear) usually recovers in one to three weeks. A partial tear takes four to six weeks, and a complete ligament rupture can take several months. Stress fractures and bone bruises can look similar to a sprain from the outside, so persistent swelling and pain after a few days is worth getting checked with an X-ray or imaging.
For the first two days after an injury, the standard approach is rest, ice, compression, and elevation. These steps limit swelling and protect the damaged ligament while it begins to heal.
Blood Clots (DVT)
A blood clot in one of the deep veins of your leg, called deep vein thrombosis, is one of the more serious causes of one-sided ankle or leg swelling. The swelling often comes on over hours or days, and you may also notice warmth, tenderness, or a dull ache in your calf. Sometimes the skin looks slightly reddish or bluish.
DVT is more likely if you’ve been immobile for long stretches (a long flight, bed rest after surgery), if you’re on hormonal birth control, or if you have a clotting disorder. Doctors take one-sided leg swelling seriously because a clot can break loose and travel to the lungs, causing a pulmonary embolism. If your swollen ankle comes with chest pain, difficulty breathing, dizziness, or coughing up blood, call 911.
To check for a clot, your doctor will typically start with a blood test that detects clot breakdown products. This test is extremely sensitive (about 96%), meaning a negative result reliably rules out a clot. If the test is positive or suspicion is high, a compression ultrasound of the leg confirms the diagnosis.
Infection (Cellulitis)
Cellulitis is a bacterial skin infection that commonly affects the lower leg and ankle, almost always on just one side. The skin becomes red, swollen, warm, and painful to the touch. As the infection progresses, you may develop fever, chills, blisters, or dimpling of the skin. A small cut, insect bite, or cracked skin between the toes can let bacteria in.
Cellulitis spreads quickly through the tissue under the skin, and the redness often expands visibly over hours. If you notice a warm, painful, spreading area of redness on your ankle or lower leg, especially with fever, it needs prompt medical attention. Treatment is a course of antibiotics, and most people improve within a few days once they start.
Gout Flares
Gout causes sudden, intense joint swelling when needle-shaped crystals of uric acid form inside a joint. Most people think of gout as a big-toe problem, but it can hit the ankle too. A gout flare often starts at night, and the pain can be severe enough to wake you up. The joint looks red, feels warm, and is extremely tender.
Flares typically affect one joint at a time and resolve on their own within a week or two. Between episodes, you may have no symptoms at all. Certain foods (red meat, shellfish), alcohol, dehydration, and some medications can trigger a flare. If you’ve had repeated episodes of sudden, intense ankle swelling that come and go, gout is a strong possibility worth discussing with your doctor.
Tendon Problems
Tendonitis in the ankle can cause swelling that sits in a very specific spot, which helps narrow down the cause. The posterior tibial tendon runs along the inner side of the ankle, behind the bony bump and down toward the arch. Inflammation here causes swelling and pain along that path, and over time it can flatten the arch of your foot.
Peroneal tendonitis, by contrast, causes swelling on the outer side of the ankle. Both types tend to develop gradually from overuse: running, walking long distances, or wearing unsupportive shoes. The swelling is usually mild compared to a sprain or gout flare, but it’s persistent and worsens with activity.
Chronic Venous Insufficiency
If your ankle has been mildly swollen for weeks or months and you notice it’s worse at the end of the day or after standing for long periods, the problem may be in your veins. Chronic venous insufficiency happens when the one-way valves in your leg veins weaken, allowing blood to pool rather than flow back toward the heart. About one in three adults develop varicose veins, and roughly one in 50 of those people eventually develop chronic venous insufficiency.
Early on, the only signs may be achy, tired legs and mild swelling. Over time, the increased pressure can burst tiny capillaries in the skin, causing reddish-brown discoloration around the ankle. If left untreated, the skin can become fragile and break down into open sores called venous ulcers. Compression stockings, leg elevation, and regular movement are the main ways to manage it, with procedures available for more advanced cases.
How Doctors Figure Out the Cause
The diagnostic approach depends on whether the swelling came on suddenly or has been building over time. Acute one-sided swelling triggers an immediate evaluation for a blood clot, starting with the blood test and ultrasound described above. If imaging shows no clot, doctors look at the joint itself with X-rays (for fractures or arthritis) or fluid analysis (for gout or infection).
For chronic swelling, a venous ultrasound with reflux testing can confirm whether the valves in your leg veins are functioning properly and distinguish venous insufficiency from lymphedema, a condition where the lymphatic drainage system is impaired. In some cases, specialized imaging can check for structural problems like a compressed vein in the pelvis.
Pay attention to the details of your swelling: when it started, whether it changes throughout the day, whether the skin has changed color or temperature, and whether you have pain at rest or only with movement. These specifics help your doctor zero in on the cause faster than any single test can.