The Turkish giants have agreed a three-year deal with the Italian trainer, who has been out of work since being sacked by Manchester City in May
The Turkish giants had been in the market for a new coach after Fatih Terim was fired following a bust-up with club president Unal Aysal.
Following a meeting on Sunday, they have moved swiftly to appoint the former Inter and Manchester City coach, who has been out of work since being sacked by the Etihad Stadium outfit in May.
The Istanbul giants confirmed the news via Twitter, saying: "Galatasaray have reached a three-year deal with Roberto Mancini beginning with the 2013-14 season."
Mancini, meanwhile, has spoken of his pride at joining Gala and is looking forward to their clash with Juventus on Wednesday.
"I'm very proud to have come to a club as big as Galatasaray," he said.
"It's different. I've made the right decision and I've come to a huge club. The Juventus-Galatasaray clash is a happy coincidence for me."
Mancini will earn €3.5 million for the duration of the remainder of the current season and €4.5m for the remaining two years, with additional incentives on offer.
The 48-year-old started his coaching career at Fiorentina, leading the Viola for one year before taking the reins at Lazio in 2002.
Two years later, he got the Inter job, masterminding three Scudetto triumphs, before lifting the FA Cup and the Premier League during a four-year stint with City.