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.
Acting FDA commissioner Janet Woodcock said that the public "can be very confident" the vaccine meets high safety, effectiveness and manufacturing quality standards.
It still has emergency use authorisation for children aged 12 to 15.
The jabs are being provided at no cost to Americans.
The FDA initially gave Pfizer temporary authorisation - a clearance given if the agency determines the benefits of a product outweigh potential risks during a public health emergency.
This full approval is essentially permanent. The licensing process requires companies to provide the FDA with information on how and where the product is made, as well as other clinical testing data.
Critics had been calling on the FDA to speed up this approval process as the nation struggled with dropping vaccination rates earlier this year. The spread of the contagious Delta variant has already given some hard-hit regions a bump in vaccination rates in recent weeks.