Two hundred million doses — enough to fully protect 100 million people — would be shared this year, with the balance to be donated in the first half of 2022, a source told the Associated Press.
National security adviser Jake Sullivan told reporters that Mr Biden was committed to sharing vaccines because it was in the public health and strategic interests of the US.
As the president embarks on his first foreign trip, he is aiming to show “that democracies are the countries that can best deliver solutions for people everywhere”.
“As he said in his joint session [address], we were the ‘arsenal of democracy’ in World War II,” Mr Sullivan said. “We're going to be the ‘arsenal of vaccines’ over this next period to help end the pandemic.”
The news of the Pfizer sharing plan was confirmed to the Associated Press by a person familiar with the matter and was first reported by the Washington Post.
The US has faced mounting pressure to outline its global vaccine sharing plan.
Inequities in supply around the world have become more pronounced, and the demand for shots in the US — where nearly 64 per cent of adults have received at least one dose — has dropped.
The announcement comes a week after the White House unveiled its plans to donate an initial allotment of 25 million doses of surplus vaccine overseas.
These will be distributed mostly through the United Nations-backed Covax program, promising infusions for South and Central America, Asia, Africa and others at a time of shortages abroad.
Overall, the White House has announced plans to share 80 million doses globally by the end of June.
Officials say a quarter of the nation's excess will be kept in reserve for emergencies and for the US to share directly with allies and partners.
The White House has also directed doses to allies including South Korea, Taiwan and Ukraine.
Global public health groups had been aiming to use the upcoming G7 meetings in Cornwall to press the nation's wealthiest democracies to do more to share vaccines with the world.
“The Biden administration's decision to purchase and donate additional Covid-19 vaccine doses is the kind of bold leadership that is needed to end this global pandemic,” said Tom Hart, acting CEO at The ONE Campaign, a nonprofit that wants to end poverty.
Additional reporting by the Associated Press.