Jacob Zenn, a terror expert who lives in Nigeria, told CNN on Saturday the alliance would make sense for both groups.
"Boko Haram will get legitimacy, which will help its recruiting, funding and logistics as it expands," Zenn said. "It will also get guidance from ISIS in media warfare and propaganda. Previously Boko Haram was a sort of outcast in the global Jihadi community. Now it is perhaps ISIS's biggest affiliate.
"ISIS gets more international legitimacy as a global caliphate."
Boko Haram, whose name translates as "Western education is sin," has been waging a yearslong campaign of terror aimed at instituting its extreme version of Sharia law.
Boko Haram's tactics have intensified in recent years, from battling Nigerian government soldiers to acts disproportionately affecting civilians -- such as raids on villages, mass kidnappings, assassinations, market bombings and attacks on churches and unaffiliated mosques.
Much of this violence has taken place in Nigeria. But neighboring countries, such as Cameroon and Chad, have also been hit increasingly hard.