The 24-year-old has reportedly penned a four-year deal with the Blades who sealed Premier League promotion at the end of last season.
Auston started his career in the United States with Bethlehem Steel before moving to Philadelphia Union in 2017.
He signed with Arsenal in 2022 after a stint at Colorado Rapids where he made 57 appearances in Major League Soccer.
The Gunners sent him on loan to Birmingham City for the 2022-23 campaign, where he was impressive with his performances.
The young centre-back helped the Blues finish 17th in the Championship table last season. He started 44 league matches, netting four goals.
His excellent displays saw him named Birmingham’s Player of the Year last term.
Trusty also received an international call-up by the US in March, starting their Nations League game against Grenada.
He was part of Arsenal’s pre-season tour, featuring against Nuremberg and MLS All-Stars.
However, he has now completed his transfer to United as their third signing of the summer transfer window.
The Blades sealed Premier League promotion after finishing second in the Championship last season.
They have returned to the English top flight after suffering relegation to the second-tier at the end of the 2020/21 campaign.
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