Trillian Capital Partners has been ordered to pay back Eskom almost R595-million by the High Court in Pretoria.
The ruling was handed down on Tuesday.
Approached for comment, Eskom said it would need to refer to its legal team for more information.
The court ruled that payments from the State-owned power utility to Trillian were "unlawful and invalid".
Eskom launched a bid to set aside the payments to global consultancy McKinsey and Trillian in March 2018.
McKinsey apologised and paid back R902-million to Eskom in early July 2018.