The Food and Drug Administration (FDA) authorised the US-made jab about a week after approving a Pfizer/BioNTech vaccine which is now being distributed.
The US has agreed to purchase 200 million doses of Moderna, and six million may be ready to ship now.
The country has the world's highest numbers of Covid-19 deaths and cases.
It has recorded at least 311,529 deaths and 17,269,542 infections, according to Johns Hopkins University.
"To go from having a sequence of a virus in January to having two vaccines available in December is a remarkable achievement," said Dr James Hildreth, CEO of Meharry Medical College in Tennessee.