Spithill, whose US outfit suffered a heavy defeat
by Emirates Team New Zealand in Bermuda, previously raced for the Italians during the America's Cup cycle in 2007.
He earned the nickname "James Pitbull" for his aggressive tactics in taking Luna Rossa to the final of the challenger series before losing 5-0 to the Kiwis in Valencia, Spain.
"With his huge sports and technical experience on high-performance sailing boats, Jimmy brings additional strength to team Luna Rossa," said the syndicate on its website.
Spithill made his debut in the America's Cup in 2000 and became the then youngest skipper to win the prestigious Auld Mug when he led America's BMW Oracle Racing to victory aged 31 in 2010.
He defended it three years later with that remarkable comeback to win 9-8
against New Zealand in San Francisco, but lost his crown when 26-year-old Peter Burling steered the Kiwis to a resounding 7-1 win on Bermuda's Great Sound in June.
"The America's Cup is a technology game and really the big question is how hard you push things, how extreme do you go on some of your design and engineering systems?," the 38-year-old Spithill told CNN Sport.
"One team were a little bit more extreme and pushed the boundaries more.
"When I look back when we won the America's Cup the previous two occasions, we were similar. We pushed as well. So potentially, maybe the fact that we won the first two made us a little bit more conservative."
Beating the bullies
Growing up in remote Elvina Bay, 35 kilometers north of Sydney, Spithill sailed from an early age, but he recently spoke of how being bullied as a child inspired him to succeed.
"This bullying was a bit tough for me to cope with then, but in the long run I think it made me a better person, giving me a thicker skin and greater determination to succeed," he wrote in "Chasing the Cup: My America's Cup journey."
Luna Rossa withdrew from the 2017 America's Cup campaign because of a disagreement over rule changes, but the Italian syndicate was first to challenge the victorious Kiwis to the next match, earning the title Challenger of Record.
The 36th America's Cup will be held in New Zealand in March 2021.
Instead of the foiling mutilhulls with rigid wing sails of the last two events, the competition will be sailed in 75-foot foiling monohulls.