The 18-year-old Turkey international joins on a one-month deal after injuries to Bellamy and Smith leaves Malky Mackay short of attacking options for Saturday’s tie with Burnley
The 18-year-old made 25 appearances for the Cottagers last season after breaking into the first team, but a pelvis injury has hampered his chances of further progression in west London his term.
Bluebirds manager Malky Mackay has been forced to seek cover after injuries to Craig Bellamy and Tommy Smith have left him short of attacking options, telling reporters that the pair are both “a couple of weeks away”.
Cardiff were granted international clearance in order for the winger to be available for Saturday’s Championship clash at home to Burnley.
And Frei, who switched international allegiances from Switzerland to Turkey in June, is set to obtain a good level of experience during his time at the Cardiff City stadium.
Frei wrote on his Twitter page: "Happy to be on Loan at Cardiff City! Can't wait for tomorrows game against burnley!"
The Welsh side are currently level on points with Leicester at the top of the Championship and recently managed a club record six successive home wins.