Stringer, who departs GWS after previous stints at the Western Bulldogs and Essendon, had already been training at the Saints before Wednesday's official confirmation.
The 250-gamer nicknamed The Package was delisted by the Giants after expressing a desire to return to Melbourne and be closer to family.
Stringer, despite a history of injuries, will add firepower to the Saints as a seven-time club leading goalkicker - three each at the Dogs and Bombers, and last season with the Giants.
Meanwhile, Elijah Tsatas, taken by Essendon with pick five at the 2022 national draft, has moved to North Melbourne.
Tsatas joins ex-Richmond ruckman Samson Ryan and Irishman Darragh Joyce, who had played for St Kilda and Brisbane, at the Kangaroos.
"With a slightly more compressed draft hand this year, we made the strategic decision to be very active in the delisted free agency period," North's head of football talent Brady Rawlings said.
"Elijah already has a solid amount of experience under his belt but we know he's still got plenty of improvement ahead of him.
"Samson has six years' experience in the AFL system and will give us added depth among our talls ... and Darragh provides proven reliability and key-position flexibility that will strengthen our back half."
Wil Parker's shift from Collingwood to Carlton in the delisted free agency period that ends on Friday has also been rubber-stamped.
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