Dr Ramos-Horta, who was Timor-Leste’s president from 2007–2012, said he has spent months considering whether to run but has decided against it.
"I have received a lot of support, appeals from across the country, political leaders, the church, led to me reflect on that ­ but I decided not to seek another term," Dr Ramos-Horta said.
Recently Timor-Leste’s former prime minister, Xanana Gusmao, threw his support behind the Fretilin party candidate, Lu'Olo, but Dr Ramos-Horta said that was not the reason he decided not to run.
"Not at all, because I never discussed with Xanana Gusmao or anyone whether they should support me or not," he said.
Dr Ramos-Horta said the powers of the president in Timor-Leste were limited and he said he would be able to make a contribution in other ways.
He said it was time to make way for new leaders, and that he would remain available to help those elected in the presidential and parliamentary elections this year.
"I will remain available to work with the new president and to look at how I can help the new government," he told the ABC.
Dr Ramos-Horta said the next five years would be crucial for East Timor.
"We will have to address serious issues to further stabilise the country, further consolidate peace, further improve our democratic institutions, eliminate corruption from the face of Timor-Leste," he said.
Timor-Leste’s President, Taur Matan Ruak, is not expected to run for a second term.
Instead, he is expected to run for prime minister for the new Peoples Liberation Party in elections later this year.