Smithies, who played close to 300 games for hometown club Huddersfield prior to joining QPR in 2015, has signed a three-year contract that will keep him in the Welsh capital until 2021.
"I wondered if he just has a blinder when he plays against us. But having watched Alex for a while, there is no doubting his pedigree," manager Neil Warnock told the club's official website.
"He's a lad that knows he will have to fight for his place. Neil Etheridge had a fabulous season in 2017/18 and improved as time went on. It's great for us to have that healthy competition."
So far this summer, Cardiff have already signed Greg Cunningham from Preston and Josh Murphy from Norwich as they look to improve on their last top flight campaign in 2013/14 and avoid immediate relegation back to the Championship.
The Bluebirds will begin the 2018/19 season away against Bournemouth on 11th August.