The court ruling adds that it considers all affiliates of the group to be under the same categorization as well, state-owned MENA news agency reported.
Ansar Beit al-Maqdis (Victors of Jerusalem), an armed group based in the Sinai Peninsula, which carries out regular attacks on Egyptian security forces and cross-border attacks against neighboring Israel, pledged allegiance to ISIL in November.
ISIL has carved out a self-styled caliphate in territory it controls in Syria and Iraq and demanded the loyalty of the world's Muslims.
A US-led coalition began targeting ISIL in airstrikes since September 23.
Saudi Arabia and the United Arab Emirates banned the group earlier this year.
In a widescale security crackdown last December, Egypt blacklisted the Muslim Brotherhood - Egypt's oldest and most influential Islamist group - after President Mohamed Mursi was toppled by the army following mass protests against his government in July 2013.
Hundreds of its members, including its top leaders, received death sentences and lengthy jail terms earlier this year. Many were later commuted to life sentences.