Food and Drug Administration (FDA) chief Stephen Hahn said: "With the availability of two vaccines now for the prevention of Covid-19, the FDA has taken another crucial step in the fight against this global pandemic."
"Congratulations, the Moderna vaccine is now available!" tweeted President Donald Trump.
The US is the first nation to authorize the two-dose regimen from Moderna, now the second vaccine to be deployed in a Western country after the first, developed by Pfizer and BioNTech.
The Pfizer-BioNTech vaccine was approved by Britain on December 2, followed by several other countries including the US last week. Less-vetted shots have also been rolled out in China and Russia.
Meharry Medical College President James Hildreth, who was part of a panel of experts convened by the FDA to discuss approval matters, said Thursday it was a "remarkable achievement" to have developed and authorized the Pfizer and Moderna vaccines within the space of a year.
Together, they offer a glimmer of light at the end of the pandemic's long tunnel.
The United States alone has seen more than 310,000 people die from coronavirus infections and is in the midst of a brutal winter surge, with nearly 115,000 people hospitalized, according to the Covid Tracking Project.
Millions of doses will now start shipping out as early as this weekend from cold-storage sites outside Memphis and Louisville, overseen by logistics firm McKesson.
From there they will be delivered to sites around the country via partnerships with FedEx and UPS.
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