Javed Hashmi, whose trial will begin Feb. 7, is president of the opposition group Alliance for Restoration of Democracy.
He was arrested last year for distributing a letter, allegedly written by army officers on official military letterhead, which was critical of President Pervez Musharraf and his policies.
If found guilty, Hashmi could be sentenced to life imprisonment.
The court, which has been set up inside a jail, heard the first case of treason against a senior opposition politician in Pakistan since that of former Prime Minister Nawaz Sharif.
Sharif was tried and convicted for hijacking and terrorism in the aftermath of the military coup that brought Musharraf to power more than four years ago.
Hashmi is the acting president of the party of Sharif, who is in exile in Saudi Arabia.