Ms Madikizela-Mandela's fraud conviction dates back to 2004
Election officials have said Nelson Mandela's former wife, Winnie, can run in South Africa's election next month, despite having a fraud conviction.
Ms Madikizela-Mandela's opponents had filed an appeal saying constitutional rules barred her from parliament.
The African National Congress (ANC), which named her as a candidate, argued that the rule did not apply because she had never been sent to jail.
The Independent Electoral Commission ruled in favour of the ANC.
According to South Africa's constitution, anyone "sentenced to more than 12 months imprisonment without the option of a fine" is disqualified from parliament until "five years after the sentence has been completed".
After an appeal in July 2004, Ms Madikizela-Mandela received a three-and-a-half year sentence for fraud - which was suspended for five years.
The Democratic Alliance and Freedom Front Plus parties lodged formal objections to Ms Madikizela-Mandela's nomination, arguing that the sentence was still active.
But in a statement, the electoral commission said: "The candidate is not disqualified from standing as a candidate in the election of 22 April 2009. The objection is accordingly dismissed."
Pieter Groenewald, of Freedom Front Plus, said the commission's decision showed it was "not prepared to make an unpopular decision".
He told the South African Press Association his party was "seriously considering" taking its objection to the Electoral Court.
The Democratic Alliance said it would wait for the commission to explain its ruling before deciding on further action.