The 23-year-old, who has penned a one-year deal, joined the Loftus Road club after 20 goals in 30 games for Harrow Borough.
He found his opportunities limited at QPR but spent spells on loan with Dagenham, Bury and Colchester.
"I first spotted Troy when he played against us for Bury last season," manager Dean Smith told the club's official website. "I've spoken to a few people about him and had some very good feedback.
"He's been with us since the start of pre-season and has displayed a tremendous attitude and work ethic. He wants to do well and play at the highest level possible."
"I've been made to feel really welcome here right from day one," added Hewitt. "There are some good, young players here and I have fitted in well."
Walsall have also signed Chelsea winger Milan Lalkovic on loan while striker Craig Westcarr has penned a one-year deal and James McQuilkin is on trial.