The 41-year-old will replace fellow Italian Carlo Ancelotti, Gattuso’s former mentor who was sacked on Tuesday.
Napoli named the famously combative former Italy midfielder Gennaro Gattuso as their new coach on Wednesday in an apparent attempt to restore discipline to a team who have lost their way after a promising start to the season.
The 41-year-old will replace fellow Italian Carlo Ancelotti, Gattuso's former mentor who was sacked on Tuesday hours after Napoli beat Belgian side Genk 4-0 in the Champions League to reach the last 16 of the competition.
"Welcome Gattuso" the club said on its Twitter feed with a picture of the new coach. Napoli, Serie A runners-up in three of the last four seasons, have dropped to seventh in the table.
Gattuso, a World Cup winner in 2006, spent 13 seasons of his playing career at Milan — several of them under Ancelotti — where he also had his first Serie A coaching experience.
He was appointed Milan coach in November 2017 and left at the end of last season with two years to run on his contract after they finished fifth in Serie A, just missing out on a place in the Champions League.
Gattuso previously coached Swiss side Sion, Greek club OFI Crete and third-tier Italian side Pisa, where he won promotion.
"It was easy to accept a job at Napoli, a great club," said Gattuso as he was officially presented. "This is a strong team with good players. Ninety-nine percent of the players are suitable for the type of football I want to play."
Gattuso said he had spoken to Ancelotti earlier in the day.
"It's not been easy. Carlo has been a like a father to me in football terms. I've called him at difficult moments and he's always been kind and polite.
"I can't be compared to him. He has won everything and I'm a young coach. I will be happy if I can do 10 percent of what he has done."
Ancelotti's dismissal followed a turbulent two months in which his side went nine matches without a win in all competitions.
At one point, Ancelotti criticised a decision by Napoli's flamboyant owner Aurelio De Laurentiis to confine the squad to a training-camp for a week.
The players broke off the camp halfway through, refusing to stay in the hotel although they continued to train.
Italian media said the players were fined between one quarter and one half of a month's salary, although the club have declined to comment on the reported sanctions.