The vaccine was made by Quebec City-based ID Biomedical, a subsidiary of pharmaceutical giant GlaxoSmithKline.
The approval was announced by the Food and Drug Administration, which regulates vaccines in the United States.
The vaccine will not be available commercially; it will be placed in the U.S. government's emergency stockpile.
The H5N1 vaccine contains a boosting compound known as an adjuvant, which allows smaller doses to be used per person.
The adjuvant, which is called AS03, was in most of the vaccine Canada used during the 2009 H1N1 flu pandemic.
However, the U.S. did not use adjuvanted vaccine during that outbreak.
It's long been assumed that if H5N1 were to trigger a pandemic, adjuvants would be needed. On its own, H5N1 vaccine does not produce a good immune response. Massive doses are needed to trigger what is thought to be a protective effect.
As the goal would be to vaccinate as many people as possible worldwide in as short a period as possible, unadjuvanted H5N1 vaccine would neither be practical nor morally defensible.
This newly approved vaccine would be pulled out of the stockpile if needed, said the director of the FDA's Center for Biologics Evaluation and Research. (Vaccines are classified as biologics by regulatory agencies like the FDA.)
"This vaccine could be used in the event that the H5N1 avian influenza virus develops the capability to spread efficiently from human to human, resulting in the rapid spread of disease across the globe," Dr. Karen Midthun said in a news release.
"Vaccines are critical to protecting public health by helping to counter the transmission of influenza disease during a pandemic."
The U.S. government has been working for years with a number of flu vaccine manufacturers, funding research aimed at increasing the rapid availability of vaccine in the event of a flu pandemic. The work is overseen by the Biomedical Advanced Research and Development Authority, a division of the Department of Health and Human Services.
"In an H5N1 pandemic, the U.S. government could have more than 35 million doses of this adjuvanted vaccine available within weeks to protect people, and GSK could produce additional doses to meet the nation's needs for this vaccine," said Dr. Nicole Lurie, the department's assistant secretary for preparedness and response.
GSK also holds Canada's pandemic flu vaccine contract.
The newly approved vaccine is not the first targeting H5N1 flu to be approved by the FDA. In 2007, the agency approved a bird flu vaccine made by Sanofi Pasteur. But that vaccine does not include an adjuvant.
The World Health Organization says that as of early October, there had been 641 laboratory-confirmed human cases of H5N1 flu, reported from 15 countries. Of that total, 380 infections have been fatal.