"In all jobs there comes a time when, if you are honest, you start lacking the enthusiasm and energy you once had," he said on Friday.
Mr Muller was the National party's agriculture and climate change spokesman and 12th-ranked member of the shadow cabinet.
The 54-year-old's looming exit is his reversal in a political career marked by his changes of heart.
First elected in 2014, Mr Muller won the leadership of the National party in 2020, toppling Simon Bridges in an Australia-style leadership coup.
After a number of gaffes - including displaying a Donald Trump MAGA hat in his office - he walked away the leadership after 54 days, citing mental health issues.
In early 2021, he announced he would not seek re-election as the Bay of Plenty MP in 2023, only to reverse that decision, saying he was "hugely invigorated" by the leadership of Chris Luxon.
On Friday, Mr Muller changed his mind again, saying he couldn't commit to another three-year term.
He denied a lack of belief in National's chances to win, saying Mr Luxon and deputy Nicola Willis "will provide tremendous leadership at a time of great challenges for our country come October".
Mr Luxon described Mr Muller as "deep-thinking and hard-working".
"Politics requires an enormous commitment to do the job well and I respect Todd's decision to call time and retire from parliament at the upcoming election," he said.
Todd McClay has been promoted to National's agriculture portfolio, while Simon Watts will take on climate change.
In other changes due to Mr Muller's retirement, Joseph Mooney will take on tourism, and Chris Penk cyclone recovery.
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