Speaking from South Africa, Mr Machar told the BBC that his rebel faction could still negotiate a peace deal with President Salva Kiir.
His statement comes despite last week's heavy fighting in the city of Malakal.
Mr Machar, who first fled to the Democratic Republic of Congo, is now being treated in Johannesburg.
In July, Mr Machar's bodyguards and President Salva Kiir's presidential guards fought each other, sparking days of violence.
Hundreds of people died and more than 100,000 fled across the border.
The fighting erupted less than a year after a peace deal was signed to form a unity government and end the civil war.
Riek Machar profiled
Speaking to the BBC's HARDtalk programme, Mr Machar said: "I'm going to return to South Sudan."
"Because President Salva Kiir doesn't want democratic and transparent and fair elections to be conducted, he attacked us, he has restarted the war.
"But I am hoping that wise leaders in the region, and in Africa, and the rest of the world will throw up a political process which will bring about peace again, and the resuscitation of the peace agreement, and the reconstitution of the transitional government of national unity."
Mr Machar also denied that claims that he was a warmonger, saying that his troops were simply defending themselves against government attacks.
President Kiir's officials accuse Mr Machar's forces of starting the attacks.
Both sides in the bitter conflict have been accused of perpetrating atrocities, the BBC's Karen Allen in South Africa reports.
More on South Sudan's crisis:
South Sudan's sacked Vice-President Riek Machar - who fled the country in August - has vowed to return, saying his credibility is intact.