Malek Adly had been held in pre-trial detention since being arrested by plainclothes police on May 5.
His defence team confirmed his release.
"The case has not been referred to trial yet," Tarek Khater, one of Adly's lawyers, told AFP.
Adly had supported protests in April against the decision to hand over two Red Sea islands to Saudi Arabia, which provoked outrage in Egypt and accusations that Sisi "sold" them in return for Saudi investments.
Police arrested dozens of activists ahead of an April 25 protest, after more than 1,000 people had chanted for "the fall of the regime" in an earlier demonstration in Cairo.
The police dispersed the April 25 protest before raiding the Journalists' Syndicate to arrest two reporters who are part of the same case as Adly.
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The three were accused of "attempting to topple the ruling system" and "spreading false news", a prosecution official had said.
Egypt later on Sunday also released one of the two journalists, Amro Badr, on bail of the equivalent of 500 euros, his lawyer Doaa Mostafa said.
Badr, editor-in-chief of the yanair.net news website, was arrested at the syndicate in early May, sparking a confrontation between the union and the interior ministry.
Three top members of the union are now standing trial for "harbouring fugitives", in reference to Badr and his colleague.
An Egyptian administrative court ruled in June that the islands of Tiran and Sanafir, strategically situated at the mouth of the Gulf of Aqaba, must remain under Egyptian sovereignty.
But the government has appealed the decision.
Sisi has defended the move, saying the islands were Saudi to begin with and were leased to Egypt in the 1950s.
The former army chief come to power after toppling his Islamist predecessor Mohamed Morsi in 2013, unleashing a crackdown on his supporters that killed hundreds of protesters and imprisoned thousands.