The 32-year-old forward has moved to the Welsh club as they bid to seal promotion to the top flight this term after going as far as the play-offs last season
The 32-year-old was deemed surplus to requirements at Loftus Road this season, with Mark Hughes having strengthened significantly over the past few months, and has brought an end to a three-year spell in west London.
The Welsh club announced in a statement: "Cardiff City Football Club are pleased to announce that the deal to bring Tommy Smith to the club from Queens Park Rangers has been completed this morning."
It was also confirmed that alongside fellow new signing Matthew Connolly, who also joined from QPR this week, that Smith would be available for selection in the club's game against Bristol City this Saturday.
The former Portsmouth, Watford and Sunderland man has been handed the No.14 shirt ahead of the campaign by his new boss Malky Mackay and he follows both Craig Bellamy and Heidur Helguson into the club this summer.