The Nigerian terror group has been trying to enforce an Islamic region in Africa's biggest country since 2009
The savage killers, who have been known to use child suicide bombers, announced the unholy alliance on their Twitter account.
In a videomessage posted to the account, the group’s twisted leader Abubakar Shekau told the world how his murderous ‘troops’ would now work with ISIS.
He said: “We announce our allegiance to the Caliph ... and will hear and obey in times of difficulty and prosperity.”
ISIS currently rules a self-declared caliphate in parts of Iraq and Syria and has previously linked with other terror groups such as al-Qaeda.
The video script identified the Caliph as Ibrahim ibn Awad ibn Ibrahim al-Awad al-Qurashi, who is better known as Abu Bakr al-Baghdadi, the leader of Islamic State and self-proclaimed caliph of the Muslim world.
Baghdadi has already accepted pledges of allegiance from other jihadist groups in the Middle East, Afghanistan, Pakistan and north Africa.
Boko Haram has been waging a six-year military campaign to carve out an Islamic state in northern Nigeria.
On Saturday, four bomb blasts killed at least 50 people in the northeastern Nigerian city of Maiduguri in the worst attacks there since Boko Haram militants tried to seize the town in two major assaults earlier this year.
There was no immediate claim for the bombings but they bore the hallmarks of the Islamist group.
Around noon on Saturday, a tricycle rider detonated a bomb when they were prevented from entering a fish market on the Baga road in the west of Maiduguri, Mohammad Ajia, a trader at the market said after fleeing the scene.
A second blast hit an area known as the Monday market shortly afterwards before a car bomb exploded by a bus station near a Department of State Security (DSS) office, according to a civilian member of a joint task force.
The number of wounded was well above 36 as more had been taken to two other hospitals, a hospital source said.
Maiduguri is the capital of Borno state and birthplace of Boko Haram, which has long coveted the city as a capital for the state it wants to create.
The militants tried to seize Maiduguri at the end of January and again in early February.
Boko Haram overran a territory the size of Belgium last year, which Nigeria's ill-equipped army has struggled to take back, and the group gained worldwide notoriety in April when its members kidnapped more than 200 schoolgirls.
This year, Chadian troops cooperating with the Nigerians have reclaimed some important towns in Borno.
The army has also been able to push the militants out of some territories in neighbouring Adamawa and Yobe states.
On Saturday, Nigeria's army said it had retaken two more constituencies in Borno state.
