The 31-year-old joins as a free agent less than a week after agreeing to terminate his contract with the Bianconeri, with Nice yet to disclose the length of his deal.
Ramsey spent three years with Juventus, joining in 2019 after his contract at Arsenal expired, but he struggled to establish himself in Italy and made just three Serie A appearances last season before joining Rangers on loan.
He featured seven times for Rangers in the Scottish Premiership, but his time in Glasgow will mostly be remembered for his crucial penalty shoot-out miss in the Europa League final against Eintracht Frankfurt in Seville, which Rangers went on to lose.
🍿 The day had started well, so why not finish it well with the teaser for another programme… pic.twitter.com/PAJ8GipTck— OGC Nice 🇬🇧🇺🇸 (@ogcnice_eng) August 1, 2022
Ramsey will be keen to get some minutes under his belt to prepare for the World Cup in Qatar in November.
He becomes the second Wales international in quick succession to move to France's top tier, with centre-back Joe Rodon joining Rennes on loan from Tottenham on Tuesday.
Nice finished fifth in Ligue 1 last season and have already added Alexis Beka Beka, Rares Ilie and Badredine Bouanani in recent weeks.
The club has also been linked with moves for Leicester City goalkeeper Kasper Schmeichel and Empoli centre-back Mattia Viti.