Frantic neighbours called the police on witnessing the mutilation by peering through the house windows.
"On the scene they found the suspect who was busy eating the heart of a person with a knife and fork," police spokesman Frederick van Wyk was quoted as saying by South Africa's daily newspaper The Times.
The suspect was arrested and is due to appear in court soon.
The woman, who is in her 50s, at the centre of the love triangle said that her former lover had visited the house in Cape Town where she was living with her current partner and they had chatted together before he gave her money to buy liquor and she left.
When she returned she found her partner, Mbuyiselo Manona, had been stabbed, Wyk said.
"The whole situation was crazy. We were shouting at him to stop, but he did not listen," one neighbour said.
"Even when the police got here. The guys were scared to go in. They had to call for back-up. You can't really blame them - how do you go into a room with someone dripping another person's blood out of his mouth?" he said.
Western Cape deputy police commissioner Sharon Jephta said the motive for the murder was "definitely a love triangle".
Clinical psychologist Ian Meyer described the act of removing Manona's heart as a "primitive symbol of triumph." Such cannibalism does exist but is uncommon in South Africa.
The neighbours informed the police that woman's former boyfriend had been hired recently to install aluminium windows at the house.
This is not the first time that the woman, was embroiled in a fatal love triangle. Her husband, a wealthy Cape Town businessman Bhekizulu Tshabalala, was murdered by her alleged lover, Xolani Hobongwana, in 1996.