The 29-year-old, who has also featured on the blindside flank for Leicester, played Super rugby with the Bulls and Cats and won two caps for the Springboks in 2002.
"The signing of Marco Wentzel, someone who Andy and I know very well from our time at Leicester, is a signal of our ambition and intent at Leeds Carnegie," former England star and now Leeds head coach Back told the official Leeds Carnegie website.
"The fact that we have been able to attract someone who has as much experience of playing Premiership, Heineken Cup and international rugby as Marco demonstrates that he recognises that ambition and is aware of the environment that is being created at Leeds Carnegie.
"We want to move Leeds Carnegie forward from a good club to a great club over the next few years and by showing that we have the ability to sign players of the quality of Marco, we believe we show that we are attracting someone who has the quality to compete at the top. Hopefully he will be the first of many to follow."
Wentzel, who has admitted he will be watching this year's Lions tour with great interest, believes he will be joining a club in Leeds that is clearly moving in the right direction.
"I am really excited to be joining Leeds Carnegie and getting the chance to work with Neil Back and Andy Key again," said Wentzel.
"I went to see a couple clubs around the Guinness Premiership and Leeds was right up there in terms of the set up but my decision was not all about facilities though.
"Having Neil and Andy there means it is not a leap into the dark for me and I know that they know what it takes to compete at the highest level.
"Hopefully the team can complete the job this season and get back into the Guinness Premiership. The first year will be tough and Leeds will have to fight for their place but after that, the systems that are in place at the club means that there are some exciting times ahead."