Mziwamadoda Qwabe pleaded guilty to murder, kidnapping, robbery and illegal possession of a firearm, the Telegraph reported Wednesday.
However, South Africa is still awaiting extradition of her British husband Shrien Dewani, accused of masterminding the November 2010 murder, set up to look like a botched carjacking.
Dewani, 28, was shot dead and her body found in an abandoned taxi in Cape Town's impoverished Gugulethu township.
Another accused hitman, Xolile Mngeni, has a malignant brain tumour that has so far prevented him from standing trial, the daily said.
Xolile Mngeni and Mziwamadoda Qwabe, both charged in murder of Anni Dewani, stand trial. Photo: AFP
Britain's High Court has temporarily halted Shrien Dewani's extradition, citing mental health grounds. Dewani claimed that he was forced out of the car during the hijacking.
According to Eric Ntabazalila, a South African prosecutor, Qwabe's plea bargain contains new information on where he was dropped off by the accused after the alleged hijacking.
Shrien Dewani, 32, accused of arranging the contract killing of his wife, is under medical treatment after being sectioned and deemed a suicide risk, the Telegraph added.
His barrister, Clare Montgomery QC, said last Tuesday that he needed "a period of calm".
She told Westminster Magistrates' Court in London July 31 that keeping Dewani under medical treatment in Britain for 12 months would increase the speed of his recovery.