On June 25, 1996, a truck bomb exploded in a military housing area known as the Khobar Towers dormitory near Dhahran.
US authorities have long alleged that the bombing was carried out by a Saudi wing of Hezbollah group, which receives support from Iran and Syria.
"The defendants also provided money, training and travel documents to Saudi Hezbollah members in order to facilitate the attacks," Lamberth wrote.
"Moreover, the sheer gravity and nature of the attack demonstrate the defendants' unlawful intent to inflict severe emotional distress upon the American servicemen as well as their close relatives."
No Iran response
The lawsuit was brought by the families of 17 of the 19 people killed in the attack. Iran never responded to the lawsuit, did not send an attorney to appear in the case and is not expected to pay the award.
Iran has denied any connection to the bombing of the Khobar Towers and has rejected US allegations of its involvement as "unfounded".
But the family members can seek payment from seized Iranian accounts.
The Iran-US Claims Tribunal, which arbitrated such issues, has long been closed to new claims so the families likely would have to seek payment through foreign courts such as Italy that have seized Iranian assets.