U.N. humanitarian chief John Holmes said the appeal is seeking money for food, water, sanitation facilities, emergency shelter and health care for those worst affected by Tropical Storm Ketsana and Typhoon Parma.
To jump-start the response, Holmes said he authorized an immediate allocation of $7 million from the U.N.'s Central Emergency Response Fund.
The appeal has already received between $9 million and $10 million, Holmes said.
In addition to U.N. efforts, he said many governments in the region and elsewhere, including the United States, are helping in different ways.
Ketsana affected 4 million Filipinos, killed almost 300 people, left about 40 missing and destroyed or badly damaged almost 40,000 houses, Holmes said. Some 300,000 are still in emergency evacuation centers, he said.
The storm lashed Manila and nearby provinces on Sept. 26, causing the worst flooding in the capital in more than four decades.
Eight days later, Parma blew across the country's mountainous north, bringing more rain to the still-sodden region.
At least 16 people died when the typhoon hit the main island of Luzon on Oct. 3, though Manila _ still awash in floodwaters from Ketsana _ was spared a new disaster.
Holmes said the U.N. will be focusing its assistance on the worst affected areas _ metro Manila and some areas to the north and south and the northern tip of Luzon.
"This is the largest flash appeal I think we've ever launched for the Philippines which is no stranger to disasters, and certainly no stranger to floods," he said.
"The appeal launched today is intended to cover the needs of a million people for three months," Holmes said.
It will be reviewed in 30 days to see if the right people, programs and areas are being targeted, he said.
A third typhoon, Melor, blew into Philippine waters Monday but shifted course and was heading toward southern Japan late Tuesday, the government's chief forecaster Nathaniel Cruz said.
About 20 typhoons and storms lash the Philippine archipelago each year.