Abdul Aziz has been under house arrest on the outskirts of Islamabad since being arrested as he tried to flee the mosque complex dressed in a woman's burqa several days before the military moved in.
Siddiqui said Abdul Aziz would be released after posting 200,000 rupees ($2,500) bail in an anti-terrorism court.
After troops took control of the complex they say they recovered weapons, including machine guns, rocket-propelled grenades, AK-47 rifles, landmines and hand grenades.
Abdul Rashid Ghazi, Aziz's brother, was among those killed in the assault.
Students of the Red Mosque had come into conflict with the authorities in Islamabad a number of times before the siege.
Music shops and a group of Chinese women accused of prostitution had been attacked by students, while the leaders had called for the overthrow of Pervez Musharraf, Pakistan's then-president.
The raid on the complex, which included religious schools, was followed by a surge in violence across Pakistan.
Nearly 800 people were killed in the following four months, many in suicide attacks and bomb blasts.