Jo Stevens will leave the post of Secretary of States for Wales as Andy Burnham prepares to unveil his prime ministerial team.
The Cardiff East MP Stevens said on X she was “proud” of her role in government.
Stevens had faced criticism from some Welsh Labour politicians for not supporting more powers for the Welsh government, such as for policing, with one former minister describing her as the most “anti-devolution” Labour Welsh secretary in half a century.
But she said Wales saw its biggest budget settlement for public services in the history of devolution while she was in office.
It is not yet known who will be replacing her.
Stevens, who took up the role of Welsh secretary when Labour came to power in 2024, said it had been “a privilege and an honour” to have the job.
She added: “Wales has seen the biggest budget settlement for our public services in the history of devolution, our steel industry saved and transformed for the future, the largest public investment in Wales in history through new nuclear at Wylfa, a generational commitment of £14bn for our Welsh railways, the best local growth fund settlement of any of the UK nations and so much more.
“I wish the new Prime Minister and my successor the very best in their new roles and know that they will do a great job, alongside our new Welsh Labour leader Ken Skates, in delivering for the people of Wales.”