The 30-year-old midfielder has become the club's third summer signing as Harry Redknapp looks to re-build the squad at Loftus Road following the club's relegation to the Championship.
Henry has struggled for form as the Black Country outfit slid into League One, but Redknapp has backed the former Stoke midfielder to help the R's return to the Premier League.
|9/2||QPR are 9/2 with Paddy Power to win the Championship
"Karl's another piece of the jigsaw that can hopefully help us build a team that can challenge for promotion.
"He's a combative midfielder and you know what you're going to get from him week in, week out. He'll be ready for the Saturday, Tuesday slogs and you know you can rely on him.
"Like Danny Simpson and Richard Dunne, he's been promoted to the Premier League before so he knows what's needed."
Henry is delighted to link up with Redknapp and has bought into the ex-Tottenham manager's vision for the club, backing the veteran coach to lead QPR back to the top flight.
"Harry Redknapp being manager was a massive pull and a big attraction," said Henry.
"I spoke to him and hearing where he wants to be I jumped at the chance of joining the club.
"I have been promoted to the Premier League before and played for a few years in the top-flight, which I thoroughly enjoyed.
"The Championship is a tough league and it sounds like Harry wants to bring in the kind of players that will get this club out of the division which is fantastic."