Motlanthe, who already sits in the cabinet, was named to take over from Mbeki until elections due around April next year during a meeting of the ANC’s parliamentary caucus, ANC MPs told Reuters on condition of anonymity.
Motlanthe is a left-leaning intellectual who has never sought the limelight.
When asked if it was true that Motlanthe was the candidate named to replace Mbeki, an ANC MP said "Yes, it is".
ANC spokesman Khotso Khumalo said parliament will vote on the president in the next few days. Khumalo declined to comment on whether Motlanthe had been named as president to take over from Mbeki.
"The political leadership is addressing the matter and getting hold of the chief justice and from then on, it will be nomination and voting, between today and Thursday," Khumalo said.
ANC leader Jacob Zuma was due to address a news conference in Johannesburg at 6:00 a.m. EDT.
Parliament convenes at 8:00 a.m. EDT and Motlanthe’s appointment may be put to a snap vote on Monday, almost certain to be officially approved by the ANC-dominated assembly.