Varicose veins, also known as varicoses, are a medical condition in which superficial veins become enlarged and twisted. Although usually just a cosmetic ailment, in some cases they cause fatigue, pain, itching, and nighttime leg cramps. These veins typically develop in the legs, just under the skin. Their complications can include bleeding, skin ulcers, and superficial thrombophlebitis. Varices in the scrotum are known as varicocele, while those around the anus are known as hemorrhoids. The physical, social, and psychological effects of varicose veins can lower their bearers' quality of life.

Varicose veins have no specific cause. Risk factors include obesity, lack of exercise, leg trauma, and family history of the condition. They also develop more commonly during pregnancy. Occasionally they result from chronic venous insufficiency. Underlying causes include weak or damaged valves in the veins. They are typically diagnosed by examination, including observation by ultrasound.

By contrast, spider veins affect the capillaries and are smaller.

Treatment may involve lifestyle changes or medical procedures with the goal of improving symptoms and appearance. Lifestyle changes may include wearing compression stockings, exercising, elevating the legs, and weight loss. Possible medical procedures include sclerotherapy, laser surgery, and vein stripping. However, recurrence is common following treatment.

Varicose veins are very common, affecting about 30% of people at some point in their lives. They become more common with age. Women develop varicose veins about twice as often as men. Varicose veins have been described throughout history and have been treated with surgery since at least the second century BC, when Plutarch tells of such treatment performed on the Roman leader Gaius Marius.^{[citation needed]}

People with varicose veins might have a positive D-dimer blood test result due to chronic low-level thrombosis within dilated veins (varices).

Most varicose veins are reasonably benign, but severe varicosities can lead to major complications, due to the poor circulation through the affected limb.

Varicose veins are more common in women than in men and are linked with heredity. Other related factors are pregnancy, obesity, menopause, aging, prolonged standing, leg injury and abdominal straining. Varicose veins are unlikely to be caused by crossing the legs or ankles. Less commonly, but not exceptionally, varicose veins can be due to other causes, such as post-phlebitic obstruction or incontinence, venous and arteriovenous malformations.