Smithies has inked a three-year deal and becomes the fourth Englishman to join the Bears for their return to the competition in Western Australia next year.
The 25-year-old's deal contains an option for two further seasons beyond that.
Smithies joined Canberra in 2024 and has become a mainstay of Ricky Stuart's forward pack, piecing together a strong 2025 campaign as the Raiders clinched the minor premiership.
It has been a quieter 2026 for Smithies, who was axed from the NRL side for round 19 but returned after a fortnight out of favour.
"The opportunity to make history and be a part of the Perth Bears was too good to refuse," Smithies said in a statement.
Smithies played 114 games for English side Wigan before his 67 at the Raiders and featured for England in last year's Ashes series loss.
He joins Penrith premiership winners Liam Henry and Scott Sorensen in Perth's inaugural forward pack, with Canterbury's Josh Curran shaping as another key man.
But the Bears' roster is still lacking in top-tier talent, with Sifa Talakai and Gehamat Shibasaki their only State of Origin players to date.
Even then, neither man played in this year's series, while Perth are yet to sign any player who was a first-choice half at an NRL club this year.
Toby Sexton, Tyran Wishart, Jack Cole and Niwhai Puru are the Bears' halves options to date.
While 19th franchise PNG can offer tax-free salaries from their first season in 2028, the NRL did not provide incentives for Perth.
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