The extension has been approved by the parliament and is scheduled to start July 10, reports say.
President al-Sisi few hours after the Islamic State group (IS) directed attacks against two Christian churches in Alexandria and in Tanta, northern Egypt, declared a three-month state of emergency and vowed to bring the perpetrators to book.
The Palm Day suicide bombings left 45 people dead.
The state of emergency gives sweeping power to security forces to limit movement, arrest and expand surveillance.
Egypt has lived several years under emergency law under ousted Mubarak. The measure was removed a month before toppled Islamist President Mohamed Morsi took office in 2012.
The law was reinstated for one month after Morsi’s ouster in 2013 by the current President who was then army Chief.
Since Morsi’s removal, Egypt has been haunted by insecurity as army and law reinforcement forces face insurgency from Islamists who vowed to avenge Morsi.