The 25-year-old made 69 Belgian Pro League appearances over two years at Gent, scoring seven goals in that time.
Last season, Bruls was loaned out to Nice, where he made 26 league starts as Claude Puel's men narrowly escaped relegation.
Bruls will make his return to Ligue 1 this season, after Rennes confirmed the move on their official website on Wednesday.
"We have followed Christian for a long time," head coach Philippe Montanier said.
"We already started discussions last spring. This is a player who is very at ease technically and that also has a lot of heart.
"He is complete and vicious in the good sense of the word. Last season at Nice, he was very consistent in his performances."
Bruls will have the chance to make his debut when Rennes get their 2014-15 league campaign under way at Lyon on August 10.