The 23-year-old represented the Gunners' only acquisition of the January transfer window but remained on loan at the Colorado Rapids until the end of June.
However, the 6ft 3in centre-back seemingly does not have a path into the Arsenal first team over the likes of William Saliba, Rob Holding, Pablo Mari, Ben White and Gabriel Magalhaes.
Trusty was expected to be sent out on loan to continue his development in Europe, and Birmingham have won the race to sign the centre-back for the upcoming season.
"It feels great! I am honoured, I am blessed, I am excited for the new chapter and the new challenges. I am just really really happy to be here," Trusty told BluesTV.
Trusty left Colorado with one goal and one assist to boast from 61 appearances for the MLS outfit, who topped the Western Conference in 2021 with the centre-back as a mainstay in the squad.
The former United States youth international - who is still waiting for his first senior cap for his country - follows the likes of Harry Clarke, Mika Biereth, Tyreece John-Jules, Karim Rekik and Nikolaj Moller out of the Arsenal exit door on temporary deals.
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