The left-footed midfielder has finally completed his long-awaited return to the club he began his career at, after passing a medical on Tuesday morning
Janssen had long been linked with a return to the GelreDome outfit, and the move has now been finalised at last after the Dutchman passed his medical on Tuesday morning.
"I am very happy that the deal has finally been completed," Janssen told reporters.
"They tend to say that things you really want always take a while, and that was definitely the case here. I want to be an important player for Vitesse in the next few years."
The 31-year-old midfielder came through the ranks of the Vitesse youth academy, but left the club for Twente in the summer of 2008.
He joined Ajax ahead of the 2011-12 campaign, and helped them to their second consecutive Eredivisie title. However, he had lost his starting berth this term, and decided to leave the Amsterdam giants.