The 79-year-old who defeated George Weah in a runoff election in November took the oath of office in an outdoor ceremony in sweltering heat in the West African nation's capital Monrovia.
Boakai then paused during his speech and aides rushed to his side and fanned him.
He resumed his speech a few minutes later, but paused again. The ceremony was curtailed and aides helped him walk away from the podium, Reuters reporters at the scene said.
Boakai's spokesperson said he had no immediate comment.
Some of Boakai's opponents had raised concerns about his age and energy during election campaigning but his team dismissed the criticism.
(Reporting by Carielle Doe and Alphonso Toweh; Writing by Anait Miridzhanian and Bate Felix; Editing by Alison Williams and Andrew Heavens)
