The Italian signed a two-year deal with the Ligue 1 side to secure his return to coaching after his departure from Leicester City.
Claudio Ranieri is back in football less than four months after being sacked by Leicester City after agreeing a two-year contract with Nantes.
The Italian was unceremoniously dumped by the Foxes in February as they battled for Premier League survival, despite having led them to a stunning league title the season before.
Ranieri had been linked with a return to English football with Crystal Palace and Watford following the end of the 2016-17 campaign, but he has opted for a move to Ligue 1.
He replaces Sergio Conceicao at Nantes after the Portuguese coach left for Porto, having guided them to seventh in the table last time out.
Ranieri has penned a deal that will run until the end of the 2018-19 campaign with Les Canaris - who had accused the French Football Federation of holding up the appointment on Tuesday - and he will hope to replicate the success he had with Leicester and lead the club into the European places.
It will be his second spell in France having previously spent two seasons with Monaco, the 65-year-old leading them back to the top flight in his first season in charge.
He then took them to second place behind Paris Saint-Germain before leaving for the Greek national team and then Leicester.