"The President has relieved the service chiefs, including the heads of the army, air force and navy of their appointments," Femi Adesina told AFP.
Chief of Defence Staff Alex Badeh and the National Security Adviser (NSA) Sambo Dasuki were also sacked, a separate statement from Buhari's office added.
"President Buhari thanks the outgoing Service Chiefs and National Security Adviser for their services to the nation and wishes them well in their future endeavours," it added.
Buratai, who comes from Borno state - the worst affected by the insurgency - has been commander of the new, strengthened regional force against Boko Haram which is due to deploy later this month.
A new commander for the multi-national force will now be appointed, Adesina said.
Rear Admiral Ibok-Ete Ekwe Ibas was appointed to head the navy while Air Vice Marshal Sadique Abubakar becomes chief of air staff.
Air Vice Marshal today Riku Morgan is the new Chief of Defence Intelligence while retired Major General Babagana Monguno replaces Dasuki as NSA.
Badeh and the former army, air force and naval chiefs were appointed by former president Goodluck Jonathan in January 2014, a month after he sacked the previous top brass.
That followed a daring raid against a military installation in the northeastern city of Maiduguri, which saw aircraft destroyed and weapons seized.
The military command has come under heavy criticism for its poor handling of a six-year insurgency that has killed 15,000 people and forced 1.5 million to flee their homes.
On the commanders' watch, the militants captured swathes of territory, including towns and villages in the remote northeast in its quest to establish a hardline Islamic state.
Boko Haram has stepped up its attacks since Buhari became president on May 29, with a wave of raids, explosions and suicide bombings against civilians.
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