The 26-year-old has put pen to paper on a three-year contract with the English club.
Forest coach Steve Cooper was delighted to secure his main defensive target this summer.
"Moussa comes to Forest with a wealth of experience in one of Europe's top competitions, having enjoyed four seasons with Mainz in the Bundesliga," said Forest boss Cooper.
"In addition to being a strong, fast and accomplished centre-back, he's captained Mainz to their best league finish in the last five years. His leadership experience is invaluable for our return to the Premier League."