The announcement follows a Friday endorsement from the FDA's advisory panel on vaccines.
The vaccine is the third approved for use in the United States to inoculate against the novel coronavirus, which has killed more than 500,000 people in the United States and 2.5 million people worldwide.
Only a few million doses will be shipped to states in the days immediately after authorization, but production is expected to ramp up in coming weeks, with 20 million doses due to be delivered by the end of March and 100 million total in the first half of the year, according to Johnson & Johnson.
The new vaccine has a lower efficacy rate -- 66% overall and 72% in the United States in preventing moderate to severe cases of COVID-19 -- than the Pfizer and Moderna vaccines already in use.
In June the FDA said it would approve emergency use authorization of any vaccine shown to be more than 50% effective in clinical trials -- a rate comparable with the seasonal flu shot released each fall.
"Don't get caught up, necessarily, on the number game, because it's a really good vaccine, and what we need is as many good vaccines as possible," Anthony S. Fauci, the government's top infectious disease expert, said Saturday. "Rather than parsing the difference between 94 and 72, accept the fact that now you have three highly effective vaccines. Period."
But it has practical advantages over the others: it can be administered in one shot and does not have to be kept frozen, making it easier to distribute to rural areas and to doctor's offices and community clinics that do not have freezers.