The race was one of the most open in years, missing the U.S. team that have dominated the event for decades but failed to make the final following a ragged run that saw them finish sixth in their semi-final.
The Azzurri took the opportunity with both hands, picking up their fifth gold from track and field at this year's Games after triumphs in the men's 100m and long jump and the double in 20 kilometres walk.
In the men's sprint relay it was their first gold medal and their first of any colour since they won bronze in 1948. Italy had also never won more than three Olympic gold medals in athletics before.
Fuelled by the three individual 100 metre medallists, Jamaica surged to an emphatic win in the women's 4x100m relay, posting a national record 41.02 seconds to finish well clear of the United States and Britain.
Elaine Thompson-Herah, who now has all three available sprint golds at Tokyo, Shelly-Ann Fraser-Pryce and Shericka Jackson delivered again, after Briana Williams had done her job on the opening leg.
It was Jamaica's first gold in the event since 2004, after back to back silvers behind the U.S. in London and Rio.