In an audio recording released by ISIS media arm Al-Furqan, Abu Mohammed al-Adnani claimed the pledge of allegiance meant that self-proclaimed caliphate has now expanded to West Africa.
Al-Adnani also urged foreign fighters from around the world to migrate and join Boko Haram.
Boko Haram's pledge came in a recording posted online Saturday, claiming to be from leader Abubakar Sheka.
The Nigerian group is waging a nearly six-year insurgency to impose Muslim Shariah law in Nigeria. It began launching attacks across the border into Cameroon earlier this year, and then struck in Niger and Chad.
In June 2014, ISIS declared an Islamic caliphate in vast swaths of territory that it controls in Iraq and Syria.
Two months later, Boko Haram followed suit, declaring a caliphate in northeast Nigeria which grew to cover an area the size of Belgium.
Members of the U.N. Security Council proposed Thursday that the international community supply money, equipment, troops and intelligence to a five-nation African force fighting Boko Haram.