The action, days before a summit with the European Union, appears to reflect determination by Ukraine to demonstrate that it can be a steward of billions of dollars in foreign aid and shed a reputation as one of the world's most corrupt countries.
It came as Ukraine has secured huge pledges of foreign weapons in recent weeks offering new capabilities - the latest expected this week to include rockets from the United States that would nearly double the firing range of Ukrainian forces.
Photographs circulating on social media appeared to show Ihor Kolomoiskiy dressed in a sweatsuit and looking on in the presence of an SBU security service officer at his home.
The SBU said it had uncovered the embezzlement of more than $US1 billion ($A1.4 billion) at Ukraine's biggest oil company, Ukrnafta, and its biggest refiner, Ukrtatnafta.
Kolomoiskiy, who has long denied wrongdoing, once held stakes in both firms, which Zelenskiy ordered seized by the state in November under martial law.
Separate raids were carried out at the tax office, and the home of Arsen Avakov, who led Ukraine's police force as interior minister from 2014-2021.
The SBU said it was cracking down on "people whose actions harm the security of the state in various spheres" and promised more details in coming days.
"Every criminal who has the audacity to harm Ukraine, especially in the conditions of war, must clearly understand that we will put handcuffs on his hands," Ukraine's security service chief Vasyl Malyuk was quoted as saying on the SBU Telegram channel.
The prosecutor general's office said the top management of Ukrtatnafta had been notified it was under suspicion, as were a former energy minister, a former deputy defence minister and other officials.
Following Wednesday's raids, the parliamentary leader of Zelenskiy's Servant of the People party, David Arakhamia, wrote on Telegram: "The country will change during the war. If someone is not ready for change, then the state itself will come and help them change."
Kolomoiskiy, who faces a fraud case in the US, has been at the centre of corruption allegations and court disputes for years that foreign donors have said must be resolved for Ukraine to win aid.
Zelenskiy, who first came to fame as the star of a sitcom on Kolomoiskiy's TV station, has long promised to rid Ukraine of so-called oligarchs but had faced accusations that he was unable to move decisively against his former sponsor.
In an address overnight before the raids, he alluded to new anti-corruption measures in time for Friday's summit, at which Ukraine is expected to seek firm steps towards joining the EU.
"We are preparing new reforms in Ukraine. Reforms that will change the social, legal and political reality in many ways, making it more human, transparent and effective," he said, promising to reveal the details soon.
Russia says it has gained momentum on the battlefield recently, announcing advances north and south of the city of Bakhmut, its main target for months.
Ukraine disputes many of those claims and Reuters could not independently verify the full situation but the locations of reported fighting clearly indicate incremental Russian advances.
Troops were fighting building to building in Bakhmut for gains of barely 100 metres a night and the city was coming under constant Russian shelling, a soldier in a Ukrainian unit of Belarusian volunteers told Reuters from inside the city.
Ukraine's general staff said late on Tuesday its forces had come under fire in Bakhmut and the villages of Klishchiivka and Kurdyumivka on its southern approaches.
