Arthur Blank - owner of NFL outfit the Atlanta Falcons - made the announcement along with league commissioner Don Garber in downtown Atlanta on Wednesday.
The franchise is the latest to be announced as part of the competition's expansion, with New York City and Orlando City set to join next year, and David Beckham's Miami-based operation pencilled in as future entrants.
Garber last year revealed plans to expand the league to 24 teams by 2020 - 19 currently take part.
"We are thrilled to welcome Atlanta to Major League Soccer as our 22nd team," said Garber.
"Atlanta is one of the largest and most diverse markets in North America and has a rich tradition in sports and culture.
"And with Arthur Blank joining our ownership group, we are adding one of the most respected owners in professional sports.
"Arthur is deeply passionate about soccer and our quest to become one of the top professional soccer leagues in the world.
"We look forward to working with him and the entire Falcons organization on achieving this important goal."
The Falcons are set to move to a proposed new stadium in 2017 and the city's MLS team will groundshare with their NFL counterparts.
Atlanta's location means they are likely to join the Eastern Conference, which is currently topped by Columbus Crew.
The move sees top-flight football return to the city for the first time since the Atlanta Chiefs' withdrawal from MLS' predecessor, the North American Soccer League, re-established as a lower-tier competition in 2009.
Atlanta has a side in the current NASL, the Atlanta Silverbacks, who are managed by former US striker Eric Wynalda.
