Nigerian last group game is on August 10, but the final and third/fourth place play-off is on August 20 - the same day Chelsea play at Watford.
Mikel, who was involved in Chelsea's first pre-season training camp in Austria last week, confirmed the news on his Instagram account today.
He said: "First part of Chelsea pre-season done. Off to Atlanta to join the Nigeria team for the Olympics 2016."
While Mikel is on the quest to win gold next month, Premier League trio Odion Ighalo, Kelechi Iheanacho and Alex Iwobi won't be as they haven't been released by Watford, Manchester City and Arsenal respectively.
Mikel's unavailability won't be too much of a blow for new coach Antonio Conte following the £32m signing of N'Golo Kante from Leicester.
The Blues were also able to convince Brazil not to pick Willian for the Olympics or team-mate Oscar.
Standard Sport revealed in April that the club wanted to have talks with Brazil over Willian's selection in particular, especially as he was being lined up to play in the Copa America as well.
The 27-year-old was named by Chelsea fans as their Player of the Season last term and there were concerns over a heavy workload.
But Willian only played in the Copa America and is now preparing for the new season with the rest of his Chelsea team-mates.