The Court for Urgent Matters has officially labelled Agnad Misr (Soldiers of Egypt) a terrorist organization.
Lawyer Ahmed Soliman, who filed the lawsuit that led to the decision, said the group had claimed responsibility for a number of terrorist attacks in Egypt.
They include a bombing near Cairo University, shootings of police officers and two bombings against police targets in Giza.
According to a government tally, almost 500 people, mostly policemen and soldiers, have been killed in suicide attacks, bombings, and shootings since the ouster of Islamist president Mohamed Morsi in July last year.
Hundreds of Islamists have been killed and thousands detained in a clampdown.
Last month, the same court officially labelled another militant group, Ansar Beit Al-Maqdis, a terrorist organisation.
The group first emerged during the January 2011 uprising which toppled Hosni Mubarak.
These groups say their attacks are revenge for the state's crackdown on Islamists.