The announcement happened following a meeting of the International Olympic Committee in Lima, Peru.
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It will be the third time Los Angeles hosts the summer Games - first in 1932 and then 1984.
Also on Wednesday, the IOC awarded Paris with the 2024 Games.
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The Olympic cauldron at the Los Angeles Memorial Coliseum was lit overnight.
Also, dozens of Southern California landmarks were lit up with Olympic colors Tuesday night in anticipation of the official announcement.
LA landmarks light up sky with 2028 Olympic colors