After IAC first removed her details from their site and then said they had "parted ways with the employee in question", Ms Sacco issued a full apology via South African newspaper The Star.
According to ABC reports, she said: "Words cannot express how sorry I am, and how necessary it is for me to apologize to the people of South Africa, who I have offended due to a needless and careless tweet.
"There is an Aids crisis taking place in this country, that we read about in America, but do not live with or face on a continuous basis. Unfortunately, it is terribly easy to be cavalier about an epidemic that one has never witnessed firsthand.
"For being insensitive to this crisis - which does not discriminate by race, gender or sexual orientation, but which terrifies us all uniformly - and to the millions of people living with the virus, I am ashamed.
"This is my father's country, and I was born here. I cherish my ties to South Africa and my frequent visits, but I am in anguish knowing that my remarks have caused pain to so many people here; my family, friends and fellow South Africans. I am very sorry for the pain I caused."
Ms Sacco's company IAC, which runs popular websites including OKCupid, The Daily Beast, Vimeo and Tinder, responded quickly following news of the "offensive comment".
"There is no excuse for the hateful statements that have been made and we condemn them unequivocally," IAC said.
"We hope, however, that time and action, and the forgiving human spirit, will not result in the wholesale condemnation of an individual who we have otherwise known to be a decent person at core."
While the story of Ms Sacco's tweet sparked a huge number of angry responses across social media, many users have said something good can come of it in the form of raised awareness for Aids charities.
At one point the Aid for Africa website actually crashed due to a traffic overload, and the address www.justinesacco.com was bought and redirected to a page where users can make a donation to the charity.
Many Twitter users also pointed to the ironic fact that Ms Sacco works in public relations for an internet company.
David Cohn wrote: "Is that tweet real? You work in PR. You shld know better RT @JustineSacco 'Going to Africa. Hope I don't get Aids. Just kidding. I'm white!'."
Another commented: "@JustineSacco Let me know how fast it takes your white ass to get fired."
Ms Sacco's Twitter account has been deleted.
While Ms Sacco's comment was widely condemned, others also rued the mob mentality of Twitter. One essay on Mashable faulted "the mob's response" and the "trial by social media".
The tweet was the latest in a string of controversial comments Ms Sacco had made on the site.
Hours before, she had called London the home of "cucumber sandwiches" and "bad teeth", while she has previously tweeted, "I like animals, but when it's this cold out I'll skin one myself for the fur" at the animal rights group Peta.
In January, Ms Sacco tweeted: "I can't be fired for things I say while intoxicated right?"
- Independent and staff reporter
