The vaccine had initially been given emergency use authorisation. Its two jabs, three weeks apart, are now fully approved for those aged 16 and older.
The approval is expected to set off more vaccine mandates by employers and organisations across the country.
It comes amid lingering vaccine hesitancy among many Americans.
In a statement, the FDA said its review for approval included data from approximately 44,000 people. The vaccine, which will now be marketed as Comirnaty, was found to be 91% effective in preventing Covid disease.