Herbst stays on at Kingspan
The 28-year-old tighthead prop joined Ulster in 2014 from Super Rugby franchise the Sharks and he has since made 42 appearances for the Irish province.
"This deal is a great boost for myself and my family," Herbst said.
"My wife and I are well settled here in Belfast and our son was born here, so it is a special place for us.
"I truly believe that the squad is really starting to develop, with improving depth across most positions. I'm looking forward to competing with the guys for the starting number 3 jersey over the next few years."
Director of Rugby, Les Kiss, added: "Wiehahn is a great bloke and I'm delighted that he's agreed to stay on with us for another couple of years.
"The market is becoming increasingly competitive and the retention of our top players is a major focus for Bryn Cunningham [Operations Director, professional team] and myself.
"Wiehahn is still relatively young for a prop and we fully expect that his best years are still ahead of him. I have no doubt that he will continue to be an important player for us.
"He is very powerful in the set piece and this has been a huge weapon for us since his arrival. His work ethic is first class and he is committed to developing his game, while also ensuring that others are too.
"With Rodney [Ah You] also competing for a starting berth, and with academy prop Ross Kane developing nicely, we are starting to look pretty well stocked at tighthead.
"We also have Ricky Lutton and Jonny Simpson on the books and it is vital for us to have that strength in depth."
