The attack reportedly took place as worshippers marked the weekly Friday prayers at the mosque in the town of Al-Rawda in El-Arish.
Reports state a few minutes after prayers finished a bomb was exploded inside the building.
However, Egypt's Ministry of Heath denied this as a rumour and clarified that 75 people were injured and that there are no fatalities.
Egyptian defense officials said the attack began with explosions and then assailants began firing with machine guns into the mosque.
Egypt has been fighting jihadist militants in the northern Sinai Peninsula since they pledged allegiance to Daesh in 2014.
No casualty figures were immediately available from the attack but eyewitnesses reported ambulances ferrying wounded from the scene to nearby hospitals.
No groups has yet claimed responsibility for the attack - but the region is rocked by extremist Islamist groups.