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:

The Iridis4 has state of the art coprocessors which can each perform a trillion calculations per second - a measurement known as a "teraflop".
The university said the new machine would allow academics to work on more projects at faster speeds.
Iridis4 will be used for subjects such as engineering, archaeology and medicine, as well as computer science.
Pro vice-chancellor Prof Philip Nelson said: "Staying ahead of the game in high performance computing [HPC] is vital to help the university stay competitive.
"Simulation and computation enabled by HPC are recognised globally as the third pillar of modern research and this investment will ensure we remain world leaders in this field."
Iridis4 will be used for a range of research, including engineering, archaeology and medicine, as well as computer science.
The world's most powerful computer is China's Tianhe-2, which can perform 33,860 trillion calculations per second.
The university said its new computer ranked among the top 10 in the UK.
The most powerful is at the Science and Technology Facilities Council in Warrington.
Others are based at the University of Edinburgh, the European Centre for Medium-Range Weather Forecasts and the United Kingdom Meteorological Office.

However, 43% of the people asked said they did believe in the resurrection - although many did not think it happened as described in the Bible.
The figures found while 57% classified themselves as Christian, fewer than 10% read the Bible and prayed regularly, or go to church at least once a month.
The Church of England's General Synod will discuss the survey in November.
The survey of more than 4,000 people was commissioned by the Church, Hope - which represents churches of all denominations - and the Evangelical Alliance - which represents evangelical Christians in the UK.
Many scholars agree that Jesus was a real man, who lived in Galilee more than 2,000 years ago, although many details surrounding his life are still debated.
But, the Church of England survey found that four in 10 people did not believe Jesus was a real person, with a quarter of 18 to 34 year olds believing he was a mythical or fictional character.
The poll was part of a wider research project looking at both practising Christians and the wider population.
After Christians, the second biggest group identified in the poll - 12% - were atheists, while 9% were agnostics, Muslims represented 3%, with Hindus and Jews both making up 2%.
English Christians are more likely than the average English adult to work in education, or professional jobs, but less likely to work in finance or insurance, the survey concluded.
South Sudan's sacked Vice-President Riek Machar - who fled the country in August - has vowed to return, saying his credibility is intact.
[NEXT_CONCEPT]
A £3.2m supercomputer, one of the most powerful in the UK, has been installed at the University of Southampton.
[NEXT_CONCEPT]
Forty percent of people in England do not believe Jesus was a real person, a Church of England survey suggests.