Onana, who moved to Old Trafford from Inter this summer, has not played for Cameroon since leaving the squad midway through their World Cup campaign in Qatar.
But the 27-year-old has now accepted a call-up, opening up the possibility that United will have to release their first-choice goalkeeper for the Africa Cup of Nations, which begins in Ivory Coast in January.
"In the world of football, as in life, decisive moments arise that require crucial choices to be made," Onana said in a statement posted on social media on Monday.
"In recent months, I have been confronted with trials marked by injustice and manipulation. Yet my unshakeable love and attachment to my homeland, Cameroon, remain intact.
"My desire to represent my country has never wavered since my youth, and this aspiration remains an inseparable part of my identity. Nothing and no one can shake this conviction. I answer my nation's call with unshakeable certainty, aware that my return is not only to honour my dream, but also to respond to the expectations and support of Cameroonians, who deserve a national team determined to shine."
Cameroon are yet to book their place for the tournament, delayed a year from 2023, and face Burundi in a final qualifier on September 12.
AFCON runs from Jan. 13 to Feb. 11 meaning Onana could miss four Premier League games.
The former Ajax stopper has not played for his country since the first game of the World Cup against Switzerland.
He was left out of the team for Cameroon's second game against Serbia before flying home early.
He announced his international retirement in December, citing a disagreement with head coach Rigobert Song.