Raynaud syndrome, also known as Raynaud's phenomenon, named after the physician Auguste Gabriel Maurice Raynaud, who first described it in his doctoral thesis in 1862, is a medical condition in which the spasm of small arteries causes episodes of reduced blood flow to end arterioles. Typically, the fingers, and less commonly, the toes, are involved. Rarely, the nose, ears, or lips are affected. The episodes classically result in the affected part turning white and then blue. Often, numbness or pain occurs. As blood flow returns, the area turns red and burns. The episodes typically last minutes but can last several hours.

Episodes are typically triggered by cold or emotional stress. Primary Raynaud's, also known as idiopathic, means that it is spontaneous, of unknown cause, and unrelated to another disease. Secondary Raynaud's occurs as a result of another condition and has an older age at onset; episodes are intensely painful and can be asymmetric and associated with skin lesions. Secondary Raynaud's can occur due to a connective-tissue disorder such as scleroderma or lupus, injuries to the hands, prolonged vibration, smoking, thyroid problems, and certain medications, such as birth control pills. Diagnosis is typically based on the symptoms.

The primary treatment is avoiding the cold. Other measures include the discontinuation of nicotine or stimulant use. Medications for treatment of cases that do not improve include calcium channel blockers and iloprost. Little evidence supports alternative medicine. Severe disease may in rare cases lead to complications, specifically skin sores or gangrene.

About 4% of people have the condition. Onset of the primary form is typically between ages 15 and 30 and occurs more frequently in females. The secondary form usually affects older people. Both forms are more common in cold climates.