Islam al-Homosi was found guilty of "harming the armed forces" after the leak of private conversations between Sisi and military officers on the Rassd information site, the officials said.
Rassd made headlines for publishing videos of private meetings between Sisi and military officers and audio leaks of media interviews with Sisi, who retired in March to stand in Egypt's May 26-27 presidential election.
Another Rassd member, Amr Salama al-Qazzaz, was acquitted, the officials said.
Egypt's new constitution adopted in January stipulates that civilians can face military trials in cases involving attacks on military personnel or military installations.
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The provision has faced stiff opposition from rights activists who argue it could violate a defendant's right to an impartial trial.
Thousands of cases involving civilians have been referred to military courts since the early 2011 uprising that toppled long-time president Hosni Mubarak.
Dozens of supporters of ousted Islamist president Mohamed Morsi who succeeded Mubarak have been referred to military trials since he was deposed by the army last July.