The 23 year-old has joined Bayer to 2023.
Sports director Rolfes remarked, "Timothy Fosu-Mensah is a technically adept, fast and physically strong defender who can be used both as a right-back and in the centre.
“With his commitment, we have not only won another top alternative for the defence after the injuries of Santiago Arias and Mitchell Weiser.
"Tim is still young and capable of development. He will also help us in that perspective."
Meanwhile, football director Voller added: "He's a valuable addition to our squad. We are facing great sporting challenges in the coming weeks. For this we need resilient players who consistently deliver a top performance.
“Timothy Fosu-Mensah meets our requirements. He has had a lot of experience in the Premier League since 2017 and has the footballing class to help us achieve our goals."