Indication came barely 48 hours after he disassociated himself from moves by some groups to draft him into the presidential race.
Many people have indicated their aspirations to run for the number one position as the date for primaries close in for political parties.
Jonathan was initially reported to have rejected the presidential expression of interest and nomination forms allegedly bought for him by the nomadic Fulani pastoralists and Almajiri communities.
He reportedly said that it was an insult for people to buy the form for him without his consent.
Hours after, the former president allegedly reconsidered his stance on the issue.
It was gathered that the former president joined the APC formally and registered at his Otuoke Ward in Bayelsa State.
Report had it that Jonathan was expected to submit the completed APC expression of interest and nomination forms bought for him today (Thursday).
A source was alleged to have said that the former president had the support of the required number of APC delegates from across the country to qualify him to submit the forms.
A source told the News Agency of Nigeria late on Wednesday that some influential African leaders called Jonathan earlier on Monday to advise him to contest the election.
The source was quoted as saying that “At least three top African leaders called the former president on the matter. They all urged him to run.
“One of them specifically told him that it did not make sense travelling all over Africa settling disputes only to shy away from leadership responsibility in his home country.
“Another reminded him of the implication of failing to put the experiences he garnered as a former Nigerian president and as a continental statesman to good use.”
Jonathan lost power to President Muhammadu Buhari in the 2015 presidential election.