"He is the fourth suspect in the alleged criminal syndicate," said Adrian Lackay, spokesman for the South African Revenue Service.
The suspect's name cannot be revealed until he appears in court on Wednesday, but he is believed to have played a role in a scam run by Thai national Chumlong Lemtongthai, who was charged last year with poaching rhinos on bogus trophy hunts.
"At the moment, charges are still being formulated" with prosecutors, Lackay said.
The latest suspect was arrested Tuesday at the Emperor's Palace casino near Johannesburg's main airport, with 21,000 rand ($2,770, 2,080 euros) found on him.
A third Thai national and a South African farmer have already been charged in connection with the case.
The revenue authority says Chumlong, who is awaiting trial in Johannesburg, would obtain trophy hunting permits and then buy the rhino horns from the hunters to ship overseas.
Local media report that Chumlong would pay friends, strippers and prostitutes to pose as hunters and export the horns under trophy permits, a limited number of which are issued each year.
The animals' distinctive horns are hacked off to be smuggled to the lucrative Asian black market, where the fingernail-like substance is falsely believed to have powerful healing properties.