He says in a statement that the role has taken a heavy toll on him and his family, and that the situation has become unsustainable from a health perspective.
Who will now lead the right-wing National Party in the election on September 19 is unclear. The main opponent in the election is ruling Labor, led by the popular Prime Minister Jacinda Ardern.
Ardern herself commented that she wished Muller and his family good luck.
"No matter which side of Parliament you sit on, politics is a difficult place," the Prime Minister was quoted as saying by the New Zealand Herald.