Mark Rutte, the long-serving Dutch prime minister, was formally named the new secretary general of NATO on Wednesday, putting an experienced, strongly pro-Ukraine leader with a reputation for conciliation at the head of the alliance.

Mr. Rutte, 57, will take over from Jens Stoltenberg on Oct. 1, at a difficult time for NATO in the face of Russia's war against Ukraine and in the midst of a tight race for the American presidency that could bring Donald J. Trump, who disparages the alliance, back into power.