What causes swollen ankles?

Several things: Painless ankle swelling is most likely secondary to fluid retention (edema) which itself has many causes. Painful or tender ankle swelling is more likely an inflammatory process. Have a doctor evaluate the issue.

So many: Things can cause ankle swelling a will list a few. Cardiovascular disease, hypertension , injury, smoking, high alcohol consumption , to much salt in diet, peripheral artery disease, certain medications. I would start with interenest and maybe a referral to cardio vascular surgeon.

Various causes: There are very many causes, from congestive heart failure, to lymphedema, to venous insufficiency, blood clots, pregnancy, a various other causes, including serious and not-so-serious causes. The fact that you legs are the lowest part of your body, fluid have tendency to pool down there, with increased hydrostatic pressure -- sometime called dependent edema. See your doctor for evaluation.