The 21-year-old is being treated in hospital, where he has undergone surgery after the attack on Tuesday evening.
The man had had an argument with four men outside a popular nightclub, whom he accused of harassing his girlfriend and trying to grope her, Florentine daily La Nazione reported. One of these four men is then alleged to have attacked the American, stabbing him twice in the shoulders.
Representatives of the Careggi hospital said that the victim had responded positively to treatment and, despite his serious injuries, was "not in life-threatening condition".
The American has not yet been publicly identified, but came from New York, according to La Repubblica.
Police are reviewing CCTV footage from the nightclub and surrounding area in an effort to identify the perpetrators, and are treating the case as "aggravated injuries", which carries a jail sentence of up to seven years.