The 28-year-old Giakoumakis comes to Major League Soccer from Celtic FC, where he led the Scottish Premiership in goals and helped the team win both the league title and Scottish Cup.
Giakoumakis will take a designated player and international roster spot, essentially taking over for Martinez after the leading scorer in franchise history was let go by United and signed with Inter Miami.
The signing ends a weeks-long hunt of Giakoumakis that included reports of him heading to a Japanese team.
In the end, Atlanta got a player it desperately needed to bolster a front line that lacked offensive punch with the season opener just 2 1/2 weeks away.
"Giorgos is a player who's been on the club's radar for a while and we're excited to bring him to Atlanta," technical director Carlos Bocanegra said in a statement. "He's a powerful striker who loves to get in the box and score goals. He plays with high intensity, works hard and is someone we think will help the team both in the attack and leading the line defensively."
Giakoumakis was the Scottish league's top goal scorer with 13 goals during the 2021-22 season, and he has tallied six goals in 19 appearances this season with Celtic again sitting atop the standings.
Across all competitions, he scored 26 goals in 57 appearances for the Scottish powerhouse.
After starting his career in his native country, Giakoumakis moved to VVV-Venlo in the Dutch Eredivisie in 2020.
He had a breakout year in the Dutch top flight with a league-leading 26 goals in 30 appearances, which led to him joining Celtic the following year.
