Eyewitnesses reported ambulances ferrying casualties from the scene to nearby hospitals after the attack on Al Rawdah mosque in Bir al-Abed, west of Arish city.
President Abdel Fattah al Sisi convened an emergency security meeting soon after the attack, state television reported.
Egypt's security forces are battling an Islamic State insurgency in north Sinai, where militants have killed hundreds of police and soldiers since fighting there intensified over the last three years.
Militants have mostly targeted security forces in their attacks, but have also tried to expand beyond the peninsula by hitting Egyptian Christian churches and pilgrims.