The UN's top climate diplomat, Christiana Figueres, has said she will leave her post in July.
Ms Figueres said she would not accept an extension of her appointment which finishes this summer.
As executive secretary of the UN Framework Convention on Climate Change, she played a key role in the talks that lead to the Paris Climate Agreement.
Her contribution to the negotiation process was praised as "really extraordinary".
Ms Figueres became executive secretary in the wake of the Copenhagen climate conference in 2009, widely perceived to have been a failure.
Over the past six years she helped rebuilt the momentum for a global compact, which saw 195 nations sign the Paris Climate Agreement after weeks of intense negotiations in the French capital last December.
In her letter to the UN confirming that she was standing down, the 59-year-old Ms Figueres highlighted the importance of that deal.
"The Paris Agreement is a historical achievement, built on years of increasing willingness to construct bridges of collaboration and solidarity. It has been an honour to support you along this path over the past six years," she wrote.
Before taking the UN post in 2010, Ms Figueres had been part of Costa Rica's climate negotiating team since the mid 1990s.
Her contribution to the successful outcome of the talks in Paris was praised by climate economist Lord Stern.
"Christiana's contribution to international climate negotiations over the [past] six years has been really extraordinary," he said.
"She is gifted with an outstanding ability to see where we need to go as a world and to bring people together.
"Christiana is one of the great leaders of our time. She no doubt has much more to contribute in the coming years. The challenge for everyone is to build on her achievements, and I am sure she will be part of that."
Ms Figueres has announced her decision to stand down in the same week as the President of the Paris conference, Laurent Fabius, announced he was stepping aside from that role. He is being replaced by French environment minister Segolene Royal.