Palestinian prisoners in line with a truce deal that saw 13 Israeli hostages freed by militants in Gaza hours earlier.
Qadura Fares, who heads an advocacy group for prisoners, said 33 prisoners freed in the West Bank were handed to a team from the International Committee of the Red Cross.
He said the remaining six were being freed from a Jerusalem lockup.
The releases on both sides were part of a deal for a four-day truce in the Israel-Hamas war that began Friday.
Over the next four days, 150 Palestinian prisoners and 50 Israeli hostages are to be freed.