The 29-year-old has signed a two-year contract that will keep him at Ajax until the summer of 2013.
Ajax have paid a reported fee of €3.5million for the highly-rated midfielder.
“I want to become champions again,” said Janssen. “I leave Twente with a good feeling, they are a top club. They will be at the top again next season.
“When I arrived there, I had a more controlling role but under Preud’homme I was given more freedom. He [Preud’homme] said from the first day on, ‘You will become the best player in Eredivisie’.”
