The terrorist organization said that a Daesh-affiliated group was responsible for the attacks in Tanta and Alexandria.
Bombs tore through two Egyptian churches in different cities as worshippers were marking Palm Sunday, killing at least 37 people and wounding around 100.
In the first, attack, a bomb exploded at Saint George church in the Nile Delta city of Tanta, killing at least 26 people and wounding over 70, officials said.
Later, an explosion hit Saint Mark's Cathedral in the coastal city of Alexandria, the historic seat of Christendom in Egypt, killing at least 11 people and wounding 35 just after Pope Tawadros II finished services. His aides later told local media that he had escaped unharmed.
Daesh claimed the attacks via its Aamaq news agency, after having recently warned that it would step up attacks on Egypt's Christians, the Associated Press reported.
The blasts came at the start of Holy Week leading up to Easter, and just weeks before Pope Francis is due to visit the Arab world's most populous country.
The bombings add to fears that extremists who have long been battling security forces in the Sinai Peninsula are shifting their focus to civilians.