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.
He has some big shoes to fill in Atlanta.
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 pursuit 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 for offensive punch with the season opener just 2 1/2 weeks away.
