Sy is a given name, nickname/hypocorism (often of Seymour) and surname which may refer to:
Sydney ( ( listen)) is the state capital of New South Wales and the most populous city in Australia and Oceania.
Systemic lupus erythematosus (SLE), also known simply as lupus, is an autoimmune disease in which the body's immune system mistakenly attacks healthy tissue in many parts of the body.
Michael Sylvester Gardenzio Stallone (; born July 6, 1946) is an American actor, film director, screenwriter, and producer.
Sylvia Plath (; October 27, 1932 – February 11, 1963) was an American poet, novelist, and short-story writer.
Roger Keith "Syd" Barrett (6 January 1946 – 7 July 2006) was an English singer, songwriter, and musician.
Synesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.