The 21-year-old defender was on loan at VfL Bochum last season, where he made 21 appearances and scored five times.
Speaking to Cardiff's website, Osei-Tutu said: "I came here because it is a big club giving me a big platform.
"I thank the manager for having the belief to bring me to such a wonderful club and all I want to do is repay him. I'll do that by showing what I can do, helping the team win games and hopefully pushing for promotion.
"I have good knowledge of playing around the pitch, but right-back is my spot. I like to get up and down the pitch, attack and defend. I can't wait to get started."