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Pacquiao, a Philippine senator who has been dividing his time between politics and fighting, made the announcement in a 14-minute video posted on his official FB page.
The judges scored the fight 115-113, 116-112, 116-112 in favour of Ugas, who controlled the second half of the contest.
The venue is expected to be the MGM Grand in Las Vegas.
A rematch had been speculated for years after the two met in a ballyhooed 2015 fight.
Since Tyson hinted at a return, challengers have been lining up for a shot at the boxer known as 'Iron Mike'.
Two judges scored it 115-112 to Pacquiao and another gave it 114-113 to Thurman, who suffered his first defeat in 30 fights.
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