A 20-year-old man who was stabbed while getting his hair cut in Sydney's west has died in hospital, and a teenager has been charged with murder.

Emergency services were called to the hair salon on Mimosa Road, Bossley Park, at 12.38pm on Friday to reports a man had been stabbed by an "unknown offender".

A spokesman for NSW Ambulance said four crews attended the scene and found the man in cardiac arrest on the pavement outside the salon when they arrived.

He had a single stab wound to the right side of his chest, the spokesman said.