The appeal comes two weeks after Israel and Hamas ended 50 days of bloodshed that killed more than 2,100 people, mostly civilians, and a month ahead of a donors' conference in Cairo.
"The scope of damage and devastation is unprecedented in the Gaza Strip," James Rawley, the UN's humanitarian coordinator for the Palestinian territories, said in a statement.
"The crisis is far from over."
Rawley and Palestinian deputy prime minister Mohammed Mustafa outlined the humanitarian needs for post-war Gaza, calling for $551 million (427 million euros) for food aid, access to clean water, healthcare and education.
"An immediate measure is to end the blockade on Gaza and ensure our people never again experience the horrors of this summer," he said.
Rawley echoed calls for a "full lifting of the blockade."
Israel agreed to ease restrictions on goods entering Gaza under a truce deal reached with Hamas on August 26.
But restrictions remain on building materials, which are crucial for reconstructing the Strip, large residential parts of which were flattened by Israeli artillery and aerial bombardment during the conflict.
Of the three crossings into the tiny coastal enclave, Israel controls two -- one for personnel movement and another for goods. Egypt controls the third.
Tight restrictions of people movement are enforced, with only some in the Strip allowed to enter and exit, mostly for humanitarian reasons such as emergency medical care.
The UN has predicted that more than 100,000 people are to remain homeless in the long term.
Israel launched its Gaza offensive on July 8, ostensibly in response to rocket fire from Hamas and other Gaza-based militant groups.
Israel says it killed nearly 1,000 militants.
On the Israeli side, 67 soldiers and six civilians were killed. Most rockets fired into Israel were intercepted or struck open ground.