Before Thursday's vote, Stoltenberg had told The Guardian that the UK staying in the EU was "good for NATO" and is key to fighting terrorism and illegal mass migration.
The U.K., a nuclear power that is one of NATO's major powers, is also one of only five of the alliance's 28 members that meets a goal of spending at least 2 percent of its gross domestic product on defense.
Stoltenberg said the U.K. is the biggest force provider among European members and that the U.K. acted as a bridge between the EU and NATO.
"A strong UK in a strong Europe is good for the UK and it’s good for NATO, because we are faced with unprecedented security challenges, with terrorism, with instability and an unpredictable security environment, and a fragmented Europe will add to instability and unpredictability," he told the paper.
"So it matters what the UK does and for NATO it is an advantage to have UK leadership inside the European Union being a strong advocate for transatlantic cooperation and also for EU-NATO cooperation," he said.
Stoltenberg on Friday said NATO remains "committed" to closer cooperation with the EU and said cooperation would be stepped up at the upcoming Warsaw Summit in July.
"Today, as we face more instability and uncertainty, NATO is more important than ever as a platform for cooperation among European Allies, and between Europe and North America," he said.
united and determined NATO remains an essential pillar of stability in a
turbulent world, and a key contributor to international peace and