The former Manchester City and Sporting Lisbon midfielder spent last season at Freiburg and returns to Ligue 1 for the first time since his three-year stint at Saint-Etienne.
Fernandes, 27, is one of many new players to arrive at the Stade de la Route de Lorient after the club finished 12th last term.
He joins the likes of Edson Mexer, Christian Bruls, Sanjin Prcic and Axel Ngando in bolstering Philippe Montanier's squad ahead of their league opener at Lyon on Sunday.
Fernandes helped Switzerland reach the round of 16 at the World Cup in Brazil and said he was now eager to get started at Rennes.
"I'm glad it's finally sorted," he told the club's official website.
"It was my choice to join Stade Rennais. This is a challenge that excites me and I am ready to meet the challenge.
"This is a very good team that is far from having exploited its potential."
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