It is the first time the Senate and House of Representatives have combined to reject a veto from the United States President Barack Obama.
Both houses of the United States Congress voted to override the veto of legislation known as the Justice Against Sponsors of Terrorism Act.
The Obama administration had lobbied hard, but the Senate, particularly, has reacted with a resounding vote in favour of the act.
"On this vote, the yeas are 97, the nays are 1. Two thirds of the senators voting, a quorum being present, having voted in the affirmative, the bill on reconsideration is passed, the objections of the president of the United States to the contrary notwithstanding."
The override is seen as a humiliating blow for President Obama.
But more importantly, the act will now become law, allowing families of 9/11 victims to sue Saudi Arabian officials over the deaths.
Many supporters have campaigned for the law, convinced the Saudi government had a role in the attacks in New York and elsewhere in 2001 that killed almost 3,000 people.
New York Democratic senator Chuck Schumer says countries should be held accountable.
"Do we really want it established inflexibly in precedent that foreign countries directly responsible for financing terrorist acts on US soil are beyond the reach of justice? I don't think so. I don't think that, in an age where we have state sponsors of terrorism, I don't think that's what the Foreign Service Immunities Act ever intended."
But President Obama has called a move a dangerous, political vote.
"It's a dangerous precedent, and it's an example of why, sometimes, you have to do what's hard. And, frankly, I wish Congress here had done what's hard. I didn't expect it, because, if you're perceived as voting against 9/11 families right before an election, not surprisingly, that's a hard vote for people to take. But it would have been the right thing to do."
He says he believes it is a mistake, suggesting US men and women in uniform around the world could potentially start seeing themselves subjected to reciprocal laws.
"The concern that I've had has nothing to do with Saudi Arabia, per se, or my sympathy for 9/11 families. It has to do with me not wanting a situation in which we're suddenly exposed to liabilities for all the work that we're doing all around the world."
Although some senators expressed their reservations about the implications of the legislation, Republic senator John Cornyn from Texas says the veto will send a strong message.
"At a time when international terrorism is spreading, FBI Director Comey yesterday warned of a terrorist diaspora. The Justice Against Sponsors of Terrorism Act will send a strong message to those who sponsor terrorist attacks on American soil, including foreign governments, will answer to those victims and pay for the death and destruction that they cause."
