The 23-year-old is a right-back who can also play at the heart of the defence and he completed his medical on Saturday morning before watching Cardiff's 0-0 Premier League draw with Everton.
The France U21 international has signed a four-year contract and is believed to have cost somewhere in the region of £2m.
Cardiff boss Malky Mackay confirmed: "We have signed Kevin Theophile-Catherine from Rennes today.
"He is a good defender, he is someone we have tracked over the last year in France. He is someone who has pace, is aggressive, and has good experience in the French top division, so he is someone we feel will be a good addition to our squad."
Newly promoted Cardiff have spent over £30m this summer, having already brought in Gary Medel, Steven Caulker, Andreas Cornelius, John Brayford, Simon Moore and Maximiliano Amondarain.