Despite Republican criticism, Pelosi insists the bill is "coronavirus-centric".
Biden signed the measure in the Oval Office just hours before he is set to deliver a prime-time televised address, during which he said he plans to "talk about what we've been through as a nation this past year".
This 3rdround of stimulus is the largest amount Americans could get, but because of a change in the Senate's version of the bill, fewer Americans could get a check. Others may receive paper checks or prepaid debit cards in the mail.
The bill will direct a new round of aid to Americans - in the form of direct payments, extended unemployment benefits, an expanded child tax credit and more - nearly a year after the pandemic first upended daily life in the United States.
The House of Representatives approved the package, known as American Rescue Plan, on Wednesday.
Some families who meet income requirements with two parents and two kids could see a payment for $5,600 in their bank account by Sunday.
Almost 160 million households are expected to get payments, according to White House estimates. However, this led to the $15 federal minimum wage increase being removed from the bill since not all Democrats agreed on it.
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It allows for the extension of emergency unemployment benefits into September.
The $1,400 stimulus payments will be directed to most Americans and their dependents.
The Senate's top Republican, Mitch McConnell, savaged the bill as "Democratic overreach in the name of COVID relief".
Mayor Bill de Blasio said the bill is "exactly the bold action" that's been needed to help the city move forward with its recovery plan.
As with previous payments, most people will have the money deposited directly into their bank accounts. The direct cash includes up to $1,400 per dependent, including adult dependents.
Half of America's children who are in poverty will not be in poverty, because of this bill.
The bill also includes a child tax credit that gives some families $3,000 per child per year.
The bill, which was passed on party lines, has been touted by Democrats as one of the biggest pieces of anti-poverty legislation in decades. While no GOP lawmakers in Congress voted for it, Democrats point out that some Republican state and local government officials are pressing for the aid.