The National Union of Mineworkers (NUM) wants Deputy President Cyril Ramaphosa to succeed President Jacob Zuma as leader of the ruling African National Congress (ANC), it said on Monday.
A decision has been taken "to support the candidacy of Cyril Ramaphosa for president", NUM said in a statement.
Zuma is expected to stand down as ANC president at a party conference late next year, ahead of national elections in 2019 when his tenure as the country's leader will come to an end.
Ramaphosa, a wealthy businessman and founding member of NUM, is likely to face strong competition if he does compete for the ANC leadership, including from Zuma's ex-wife, Nkosazana Dlamini-Zuma, who is currently head of the African Union.
Neither Ramaphosa or Dlamini-Zuma have said they will run.