Atlanta used its No. 1 spot in the MLS allocation ranking to acquire Guzan, who joins the expansion club on a multi-year contract.
The 32-year-old is currently the backup to Victor Valdes at Boro, and the English club will keep hold of Guzan until the current Premier League campaign is over. Though Boro's season ends in May, Guzan will likely make his Atlanta debut in July after the MLS transfer window opens.
Guzan's delayed arrival means that Atlanta appears set to begin the season with Alec Kann, Alex Tambakis and SuperDraft selection Alex Kapp as the team's three rostered goalkeepers.
Guzan will return to MLS with the expansion club nine years after leaving Chivas USA to join Aston Villa, where he spent eight seasons. Guzan earned the starting role at Villa in the 2012-13 season and mostly held on to it until the club was relegated following the 2015-16 campaign.
The Illinois native has been capped 53 times by the USA since making his debut in 2006. He and Colorado Rapids goalkeeper Tim Howard have been splitting time as the USA's starter since Howard returned from a one-year absence in 2015.