Yvo de Boer, the UN's top climate change official, says he will resign after nearly four years in the post.
His departure takes effect from 1 July, five months before 193 countries are due to reconvene in Mexico for another attempt at a global deal on climate.
Nations failed to reach a binding deal at the Copenhagen meeting in December.
Mr de Boer said he was announcing his departure now so that a successor could be found well before the Mexico meeting later this year.
The former Dutch civil servant was appointed as executive secretary of the United Nations Framework Convention on Climate Change (UNFCCC) in 2006. As the UN's climate negotiator, he was widely credited with raising the profile of climate change issues.
But suspicion and distrust between developing and industrial countries barred the way to a binding accord at the UN's climate change summit in Copenhagen in December.
In a statement, Mr de Boer said: "It was a difficult decision to make, but I believe the time is ripe for me to take on a new challenge."
Mr De Boer said he would become a consultant on climate and sustainability issues for KPMG, a global accounting firm, and would be associated with several universities.
Mr de Boer is said to be deeply disappointed with outcome of the last summit in Copenhagen, which drew 120 world leaders but failed to reach a binding global accord.
But he said the failure to secure a treaty at Copenhagen was unrelated to his decision to quit, and that he had begun looking for a new job last year, before the summit.
He told the Associated Press news agency that he believed talks were "on track", although it was uncertain whether a full treaty could be finalised at the next high-level conference which starts in November.
Mr de Boer said the accord reached in Copenhagen, brokered by US President Barack Obama, "was very significant".
But he acknowledged frustration that the deal fell short of the consensus and was merely "noted" rather than formally adopted by all countries.
"We were about an inch away from a formal agreement. It was basically in our grasp, but it didn't happen... so that was a pity," he said.
Fifty-five countries submitted pledges for curbing greenhouse gas emissions by the 31 January deadline in the "Copenhagen Accord", the document produced at the UN climate summit in the Danish capital.
At the time, Mr de Boer said the pledges would invigorate the UN process, but several environment groups said they did not go far enough. In some cases the pledges were weaker than those made before the summit.
According to AP, people who know Mr de Boer said he was more disheartened by the slow pace of negotiations than he was ready to admit.
"I saw him at the airport after Copenhagen," said Jake Schmidt, a climate expert for the US-based Natural Resources Defense Council, "He was tired, worn out."
Mr Schmidt said that the summit "clearly took a toll on him."
In 2007, during exhausting negotiations at the Bali conference, Mr de Boer left the stage in tears after being accused by China of procedural irregularities.
Before he took up his post with the UNFCCC, Mr de Boer was involved in European Union environmental policy in his role with the Dutch Environment Ministry. He also served as vice-chair of the UN Commission on Sustainable Development.
A different UN climate body - the Intergovernmental Panel on Climate Change (IPCC), which reviews climate science - has come under fire in recent months for a mistake on the melting of Himalayan glaciers and for referencing "grey literature" - a WWF report which had not been peer-reviewed.
The head of the IPCC, Dr Rajendra Pachauri, has rejected calls for his resignation.
In November, hundreds of e-mails from the Climatic Research Unit (CRU) at the University of East Anglia, UK, appeared on the web.
CRU maintains one of the world's most important datasets on how global temperatures have changed.
An inquiry is underway to consider whether the e-mail exchanges between researchers show an attempt to manipulate or suppress data "at odds" with scientific practice.