"It is of course with mixed feelings, but I feel incredibly proud to have the opportunity to go to Serie A," Hiljemark tells Elfsborg's website.
Pisa writes on its website that the contract extends until June 30, 2027, with an option.
During his playing career, Hiljemark played for Palermo and Genoa.
Previously, Alberto Gilardino was fired as the club's head coach.
Pisa is second from bottom in Serie A. Their next match is on Friday against Verona.
