Van der Vaart has joined Hamburg for a reported fee of £10million.
The Dutchman has signed a three-year contract with Hamburg.
"It feels a bit like I am coming back home," he said. "Everybody knows that I still have a strong bond with Hamburg and it is a compliment when a club wants you every year.
"This season was the right time."
"It was a very, very hectic night and I had to race to the airport this morning to get to the medical," he told Hamburg's official website.
"If everything goes according to plan, then maybe I will travel (to Bremen tomorrow), but to be honest, I have only been here a few hours and we will just have to wait and see."