Judge George Daniels in Manhattan signed the judgement a week after hearing evidence in the 10-year-old case.
The signed ruling came in a $100 billion lawsuit brought by victims’ families.
He directed a magistrate judge to preside over remaining issues, including fixing compensatory and punitive damages.
Judge Daniels signed findings of fact saying the plaintiffs had established that the attacks were caused by the support the defendants provided to al-Qaida.
It also said Iran continued to provide material support and resources to al-Qaida by providing a safe haven for al-Qaida leadership and rank-and-file members.
During last week’s hearing, September 11 victims’ families sat through a four-hour presentation from lawyers who cited evidence supporting their claims that Iran actively assisted the hijackers of planes that crashed into the World Trade Center towers, the Pentagon and a field in Pennsylvania.
Former members of the September 11 Commission and three Iranian defectors also spoke.
It would be near impossible to collect any damages, especially from the Taliban or al-Qaida.
President Mahmoud Ahmadinejad has repeatedly denied any Iranian connection in the September 11 attacks or with al-Qaida.