Ronnie Mamoepa, a spokesman for the immigration department, said Saturday that Jennifer Rovero had two weeks to leave the country, and would not be welcome back for at least two years.
"We are saying she must go," Mamoepa told The Associated Press. "The person is found in illegal possession of an illegal substance, convicted by a duly constituted court of law. So, we have to act."
Hilton had been arrested with Rovero after Friday's Brazil-Netherlands match in Port Elizabeth on suspicion of possession of marijuana, then had the case dropped at a midnight court hearing.
Rovero, a former Playboy playmate of the month, "acknowledged to be in possession of cannabis, and pleaded guilty," said Brig. Sally De Beer, a spokeswoman for South African police.
Rovero, a 31-year-old New Yorker, was sentenced to a fine of 1,000 rand ($130) or 30 days imprisonment, De Beer said.
Hilton wrote on Twitter later: "Hey guys, there's a lot of crazy rumors going around. Just want you all to know the truth. Everything is completely fine. I was not charged or arrested, cause I didn't do anything."
"Everyone was super nice and friendly to me," Hilton wrote. "I love South Africa! Such an amazing place, especially during The World Cup! Hope that clears everything up."
Hilton pleaded no contest in 2007 to alcohol-related reckless driving and was sentenced to 45 days in jail. After spending about 23 days in jail, Hilton told U.S. television host Larry King that the experience caused her to re-evaluate the role partying played in her life. She said she wanted "to help raise money for kids and for breast cancer and multiple sclerosis."