The 23-year-old, who arrived from Natal Sharks three years ago, has been a key member of the Ulster squad since making the move from Belfast.
"The contract extension has been in the pipeline for a while now and I'm delighted to get it signed. I've had a great time at Ulster so far, three years have flown by and I'm really looking forward to a fourth year," he the club's website.
"Ulster Rugby has gone through some wonderful times over the past couple of years and it's brilliant to be part of that. The players and back room staff have done an amazing job over the past three years and it's an exciting place to be.
"Everybody is really happy being part of this squad and this environment and I'm no different. We're on the right track and hopefully we can keep on going and win some silverware."
"We've done brilliantly so far this season but the next 8 to 12 weeks is obviously going to define the season. The important stuff is coming up now. We've got the massive game on the April 6 against Saracens, but there's everything to play for in the PRO12 as well."
Muller won the 2007 World Cup with South Africa.