From New Scientist:

Some 20 million years ago, the Arctic Ocean was best described as a very large lake, whose fresh water leaked southwards through a narrow strait into the Atlantic.

Then, 18.2 million years ago, something happened. Drawn by shifting tectonic plates, the strait began to widen. Slowly, over the course of hundreds of thousands of years, salt water from the Atlantic began flowing into the Arctic turning it into the ocean we know today.