The 25-year-old forward has put pen to paper on a season-long deal in the Welsh capital, where he will be reunited with former West Brom teammates Callum Robinson and Romaine Sawyers.
Grant becomes the Bluebirds' fourth new arrival of the summer, following Dimitrios Goutas, Yakou Meite and Ike Ugbo through the door at the Cardiff City Stadium.
"I'm absolutely delighted. I can't wait to meet my new teammates and the manager, and get started," Grant told the club's official website.
"I spoke with the manager, and he said nothing but good things about the club. I know how big the stadium is, and I know how good the crowd can be. I've got a few friends here as well.
"Callum Robinson and I got on really well when we were at West Brom, and we're actually still very close. He's a good friend of mine and we complement each other when we play together, so hopefully we can replicate that."
Cardiff manager Erol Bulut added: "Karlan will bring goals, speed and work rate to this team. He works hard on the pitch, not just offensively but in defence as well.
"Now, we have a few players like Karlan who can play in different positions. He can play as a winger, but we can also use him as a striker and a second striker.
"I am very excited, and happy that we have these attacking players. All of them are good goalscorers and playmakers who can make assists. They can win games and change the table by getting results for us."
Grant will join up with his new teammates for their pre-season schedule and could make his competitive debut for the club away to Leeds United on the opening weekend of the new Championship campaign next month.