Incumbent Rodrigo Duterte is barred from another term but has been picked by a rival party faction to run for vice-president, a move critics say is an attempt to cling to power.
He was selected to run alongside a close ally, Christopher “Bong” Go, but Mr Go says he does not want to succeed Mr Duterte.
While his party says it wants Mr Go to reconsider his decision, his rejection of the nomination has led to suggestions that President Duterte’s daughter Sara Duterte-Carpio could join him on the ticket.
As a boxer, Mr Pacquiao won world titles in an unprecedented eight different weight divisions. He lost his most recent bout against a Cuban opponent and said he was considering retiring.
Accepting the nomination, he said: “I am a fighter, and I will always be a fighter inside and outside the ring.”
Mr Pacquiao has pledged to campaign against poverty and corruption.
He is a popular figure in his home country but faces an uphill battle, trailing in opinion polls consistently topped by Sara Duterte-Carpio.
It is not clear which of the PDP-Laban factions will be recognised by the country’s electoral commission for the 2022 election.