Ukraine's President Petro Poroshenko has sacked Ihor Kolomoisky - the billionaire governor of the key industrial region of Dnipropetrovsk.
It comes after armed men suspected of links to Mr Kolomoisky briefly occupied the offices of a state-owned oil firm in the capital Kiev.
Mr Kolomoisky was reportedly unhappy after parliament passed legislation aimed at weakening his influence.
Correspondents say the dispute could affect the conflict in the east.
It triggered fears of a major showdown between the tycoon and the state.
In a statement released early on Wednesday, Mr Poroshenko's office named Valentyn Reznichenko as the new acting governor of the Dnipropetrovsk region.
It also quoted the president as saying during the talks with Mr Kolomoisky that the region - a centre of heavy industry - should remain "a bastion of Ukraine in the east and protect the peace".
Mr Kolomoisky - who is estimated to be worth more than $2bn (£1.3bn) - has been widely credited with helping bring order in Dnipropetrovsk and halt the advance of pro-Russian rebels further to the east.
He is also financing a number of Ukrainian battalions fighting the separatists in the Donetsk and Luhansk region.
His dismissal comes days after armed men suspected of acting on orders from the oligarch briefly seized the headquarters of the Ukrnafta energy company and its subsidiary UkrTransNafta.
The armed men - whom Mr Kolomoisky said were from a private security firm - left the buildings after an ultimatum was issued by the central government.
Mr Kolomoisky claimed the armed men had tried to ward off an illegal takeover of Ukrnafta, in which the tycoon has a 42% stake. The state owns the rest of the oil and gas giant.
This happened after Ukraine's parliament had approved amendments to a law on state-owned companies that experts said effectively removed Mr Kolomoisky's control over Ukrnafta.