Under the US anti-terrorism act, the damages are automatically tripled, meaning that the Palestinian Authority (PA) and Palestine Liberation Organization (PLO) are liable to pay more than $650 million.
Israel welcomed the decision as a "moral victory" but the Palestinians accused the lawsuit of being politically motivated and vowed to appeal.
The jury reached its verdict following two half-days of deliberations, ending a landmark trial under US district judge George Daniels in New York that lasted more than five weeks.
The six attacks killed 33 people and wounded more than 390 others between January 2002 and January 2004.
The bombings and shootings were carried out by Hamas and the Al-Aqsa Martyrs Brigades -- blacklisted as terrorist organizations in the United States -- during the second Palestinian uprising against Israel.
The 12-member jury decided unanimously that the PA and PLO were liable on 25 separate counts connected to the six attacks.
They apportioned individual damages ranging from $1 million to $25 million to Americans who were injured or lost loved ones.
The total falls well short of the $1 billion sought by lawyers for 11 plaintiff families when the trial opened in mid-January.
- 'Moral victory' for Israel -
Israeli Foreign Minister Avigdor Lieberman welcomed the decision as a "moral victory for the state of Israel and for victims of terrorism."
"Terrorism is an integral part of the very structure of the Palestinian Authority," Lieberman said.
US attorney for the plaintiffs, Kent Yalowitz, welcomed the verdict.
"This is a great day for our country, it's a great day for those who fight terror, we're so proud of our families who stood up," he told reporters.
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It remains unclear if and how the PA can pay, and it is in serious financial difficulty because of revenue frozen by Israel.
Mahmoud Khalifa, Palestinian deputy information minister, expressed dismay at the verdict and vowed to appeal what he called "baseless" charges.
He said the case was politically motivated by "anti-peace factions" in Israel to block a two-state solution to the Israeli-Palestinian conflict.
"We are confident that we will prevail, as we have faith in the US legal system and are certain about our common sense belief and our strong legal standing," he said in a statement.
- 'Exaggerated' testimony -
The plaintiffs argued that the PA and PLO be held responsible for providing material support to the groups responsible for the attacks, blacklisted as a foreign terrorist organization in the United States.
The court also heard that members of the two groups were on the payroll of the two organizations.
Israeli lawyer Nitsana Darshan-Leitner told reporters in New York that she would leave no stone unturned in forcing the Palestinians to pay.
"We're going to take steps against their assets, they have assets in the United States, in Israel. We're going to go after bank accounts and money that they are getting paid on a monthly basis in Israel, for instance."
Defense attorneys refused to comment after the verdict.
Lawyers for the PA contended during the trial that the leadership should not be held responsible for "crazy and terrible" attacks carried out by people who acted independently.
"There is no conclusive evidence that the senior leadership of the PA or PLO were involved in planning or approving specific acts of violence," lawyer Mark Rochon argued in court last week.
He said the plaintiffs "exaggerated" testimony to make the Palestinian Authority "look bad" based in part on Israeli intelligence.
The six attacks took place against Hebrew University, in Jaffa Road, King George Street, against the number 19 bus and in French Hills, a Jewish settlement in east Jerusalem.