The 72-year-old Manchin announced his decision on Tuesday after months of reflection. It means Manchin will steer clear of a potential showdown with incumbent Republican Gov. Jim Justice, who already has announced he's seeking re-election.
Earlier this year, Manchin became the top Democrat on the Senate Energy Committee, giving the coal-state lawmaker a prominent position to shape policy. But Manchin says he has become frustrated with a lack of Senate productivity and bipartisanship.
Manchin also has sparred with Justice, who was elected governor as a Democrat in 2016, then switched to the Republican Party less than a year later. Manchin had endorsed Justice for governor.