The Grand Mufti of Jerusalem said in the fatwa on Tuesday that there should be no looting or theft of anything left behind by the Israelis because everything was the property of the Palestinians.
"It is forbidden to obstruct Israel's withdrawal from the Gaza Strip or from any other occupied area from which Israel wants to withdraw. All should cooperate to ensure the withdrawal takes place," Ikrima Sabri said.
Israel's withdrawal from all 21 Gaza settlements and four of 120 in the West Bank, over a two-week period, will mark the first time that it evacuates settlements from land that Palestinians want for a state.
The evacuation is to start on 17 August.
Palestinians, largely following a six-month-old truce, see the withdrawal as a victory for an uprising since 2000.
"It is forbidden to obstruct Israel's withdrawal from the Gaza Strip or from any other occupied area from which Israel wants to withdraw. All should cooperate to ensure the withdrawal takes place"
Grand Mufti of Jerusalem
While they welcome Israeli Prime Minister Ariel Sharon's withdrawal plan, they also fear it is a ruse for strengthening Israel's hold on the biggest West Bank colonies.
Palestinians want both territories for a state.
Sharon says the plan is a "disengagement" from conflict with the Palestinians.
He has warned that there will be a heavy military response if evacuations come under fire from fighters.
The World Court calls all settlements illegal but Israel disputes this.