Tony Yengeni, the former chief whip of the ANC, agreed to the plea in exchange for his acquittal on corruption charges.
"I admit that I failed to disclose to Parliament, in circumstances where there was a duty to disclose, that I received the benefit... and that I misrepresented the facts and events as set out," Mr Yengeni said in his plea.
"The above misrepresentations were made with the intent to defraud Parliament."
He was accused of accepting a 47% discount on a luxury four-wheel-drive from Mercedes, whose US-German parent company DaimlerChrysler had stake in a firm bidding for the deal.
The case was regarded as the acid test for South Africa's anti-corruption drive.
First victim
Mr Yengeni allegedly promised to use his influence to promote a bid for the arms contract by European Aeronautic Defence and Space (EADS), a sister company of Mercedes.
Michael Woerfel, the German former head of EADS in South Africa, also faces corruption and fraud charges.
The corruption allegations surfaced in September 1999 after the deal to acquire new fighter jets, helicopters, submarines and warships valued at about 52bn rand (£3.85bn; $6.2bn) was announced.
Other weapons makers whose business practices have been inquired into as part of the investigations include Britain's BAE Systems, Sweden's Saab, France's Thomson-CSF (now known as Thales), Agusta of Italy and the German Frigate Consortium.
Legal defence
Mr Yengeni was the chairman of parliament's joint standing committee on defence at the time.
He was forced to resign as the ruling party's chief whip in October 2001 but has remained an ANC parliamentarian and party official.
In a written plea handed to the Pretoria Commercial Crimes Court, Mr Yengeni said he convinced his co-accused Michael Woerfel to arrange the car deal.