Outgoing Dutch Prime Minister Mark Rutte was officially named Wednesday as the next secretary-general of NATO, as the alliance focuses on supporting Ukraine in its war against Russian invaders and bolstering its own security needs.

Mr. Rutte, 57, will take over from Secretary-General Jens Stoltenberg on Oct. 1. Mr. Stoltenberg has led the alliance since 2014, shortly after Russia annexed Ukraine’s Crimean Peninsula.

“It is a tremendous honor to be appointed secretary-general of NATO. The alliance is and will remain the cornerstone of our collective security,” Mr. Rutte said Wednesday shortly after NATO announced his selection. “Leading this organization is a responsibility I do not take lightly.”

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NATO’s secretary-general is the alliance’s top international civil servant, responsible for chairing all major committees and ensuring that decisions are implemented. The person is also the principal spokesperson for the 32-national alliance and head of the organization’s international staff.

Mr. Stoltenberg said he strongly supported the decision to appoint Mr. Rutte as his successor.

“Mark is a true transatlanticist, a strong leader and a consensus-builder. I wish him every success as we continue to strengthen NATO,” Mr. Stoltenberg said Wednesday in a statement. “I know I am leaving NATO in good hands.”

Mr. Rutte served as the Netherlands’ prime minister for nearly 14 years. He will be introduced to other NATO heads of state and government at the alliance’s summit in Washington on July 9-11.

“I’m grateful to all the Allies for placing their trust in me. I look forward to taking up the position with great vigor in October, as successor to Jens Stoltenberg, who has provided NATO with outstanding leadership for the past 10 years, and for who I have always had great admiration,” he said.