The Bluebirds agreed a deal for the talented 21-year-old on Saturday and he is available for selection for the club's home clash with Bristol City.
Etuhu, who starred in City's run to the FA Youth Cup final in 2006, can operate as a winger or a striker.
Cardiff manager Dave Jones told the club's official website: "We are really pleased to get Kelvin - he is a Premier League player coming into the club.
"It looked like he was going to Wolves, but there was a hitch and we stepped in to bring him here.
"He is a big pacy lad who will give us options across the frontline."