Douglas Herridge, 18, hit Colin Payne at the family home in Dartford, Kent, in November.
Jurors at Maidstone Crown Court heard Mr Payne intervened in a row over money between the teenager and his mother.
Mr Herridge said the 54-year-old grabbed his neck and tried to throttle him, when he struck the blow.
Home Office consultant pathologist Dr Richard Chapman said Mr Payne, a former national squash champion, died from a blow of moderate force that damaged a vessel carrying blood from the heart to the brain.
Judge Jeremy Carey, presiding, said: "The word tragedy is probably overused, but this case can truly be described as tragic.
"Colin Payne was on everybody's assessment a fine man, a distinguished amateur sportsman, a good father, and a responsible and decent member of society, and he came to an untimely death in circumstances that really were absolutely devastating."
A teenager who fatally struck his stepfather with a single punch to the head has been cleared of manslaughter, because he acted in self-defence.