The US has acknowledged the existence of its "extraordinary rendition" scheme in the years after 9/11 but has never revealed which of its allies hosted the secret detention facilities.
Amrit Singh, a lawyer who acted for one of the men who brought the case, said the court's ruling established beyond reasonable doubt Poland hosted a secret jail run by the US government.
She said: "It's an historic ruling. It's time for them to own up to the truth."
The court case was brought by Saudi-born Abu Zubaydah and Saudi national Abd al-Rahim al-Nashiri who alleged they were flown in secret to a CIA-run jail in a Polish forest and tortured.
The two men, who are now in Guantanamo Bay, brought the case against Poland for failing to prevent their illegal detention and torture.