The newly-promoted Premier League side announced on Thursday morning that Smithies has penned a four-year deal in South Wales for a reported fee of £3m up front.
Smithies, who has more than 350 league games under his belt for Huddersfield Town and QPR, is excited by the prospect of battling it out with established stopper Neil Etheridge for regular playing time next season.
"It feels fantastic. I'm delighted to get it over the line," he told his new side's official website. "I'm looking forward to being a Premier League footballer for Cardiff City - it's a fantastic opportunity for me.
"With the atmosphere at the club at the moment, it's a great place to be. They had a fantastic season last time out and thoroughly deserved to get promoted.
"It's going to be difficult. Neil had a really good year last year, and only one player can play in goal. I'm going to do everything I can to get in the team. It will be a tough challenge but I think we will both push each other on and that will help the team."