Johannesburg - The South African Revenue Service confiscated $9.3m at Lanseria Airport, northwest of Johannesburg, from two Nigerians and an Israeli, it said on Sunday.
"The passengers' luggage was searched after customs officials detected irregularities in the luggage," spokesperson Adrian Lackay said.
"The money was detained as it was undisclosed/undeclared and above the prescribed legal limit for bringing cash into the country."
The incident happened on Friday.
He said the money was being kept safe by the SA Reserve Bank.
The matter has been handed over to the police and the national investigations unit would investigate.
National police spokesperson Solomon Makgale confirmed that the matter had been reported to the unit.
He said the money was seized and the matter handed over to the Hawks.
According to a local newspaper, the men were going to use the money to buy weapons.
The passengers were reportedly acting on behalf of the Nigerian intelligence service, but this could not be confirmed by Nigerian authorities.
The group also had documents confirming they were in South Africa to buy arms.
It was unclear whether the Israeli passenger was an intelligence operative or an arms dealer, according to the report.
The National Conventional Arms Control Committee, which approves permits for the import and export of weapons, was not aware of any applications made for such a transaction.
The committee reportedly confirmed that it was aware of the incident but did not want to comment further.