"A presidential pardon has been granted" for Zyad el-Elaimy, according to his attorney Tariq Al-Awadi in a social media statement.
The former parliamentarian, lawyer and key activist from the 2011 revolt had been jailed for the past three years was sentenced to a year in prison in March 2020 for "spreading false news" after an interview with outside media.
He is the latest in a string of prisoners to have been pardoned, with his release ordered as Egypt readies to host the United Nations COP27 climate summit next month.
Cairo has faced frequent criticism of its rights record under Sisi, who ousted late Islamist president Mohamed Morsi in 2013 and was elected president the following year.
Advocacy groups say there are currently about 60,000 political prisoners in the country, a number Cairo rejects.
Rights groups hope to raise the issue at the upcoming COP27 climate summit.
Among the best-known detainees is activist Alaa Abdel Fattah, who has been on hunger strike for more than six months..
Fattah was sentenced by a Cairo court in December to five years in prison after he was convicted along with two others of "broadcasting false news", a charge often used in recent years against activists and dissidents.