Ukraine’s Defence Minister Oleksii Reznikov was dismissed by President Volodymyr Zelenskyy on Sunday.
Mr Zelenskyy has said he will nominate Rustem Umerov, the head of Ukraine's main privatisation fund, to replace Mr Reznikov.
The announcement came as the President revealed a new training agreement with France.
Mr Reznikov, who was appointed defence minister in November 2021, has helped to secure billions of dollars of western military aid to help the war effort, but there have been allegations of corruption at his ministry.
"I've decided to replace the Minister of Defence of Ukraine," Mr Zelenskyy said in his Sunday night video address.
"Oleksii Reznikov has been through more than 550 days of full-scale war.
"I believe the ministry needs new approaches and other formats of interaction with both the military and society as a whole.
"Now Rustem Umerov should be the head of the ministry. I expect Parliament to support this candidate."
Mr Reznikov, 57, confirmed his departure in a social media post on Monday in which he said it was an "honour to serve" in the "toughest period of Ukraine's modern history".
He wrote in a resignation letter that Ukrainian counter-offensives had recaptured more than half of the territory occupied by Russia.
"Every day our defenders are moving forward," he said.
Ukrainian media has speculated that Mr Reznikov could be appointed ambassador to the UK, where he has been a familiar interlocutor for senior politicians.
The change of defence minister must be approved by parliament but is likely to be supported by most legislators in the Verkhovna Rada.
Mr Umerov, 41, a former politician who is a Crimean Tatar, has led Ukraine's State Property Fund since September 2022.
He has played a role in sensitive wartime negotiations, including on the Black Sea grain deal.
Mr Reznikov's dismissal comes after Kyiv launched its counter-offensive.
Ukraine on Sunday also fought off an attack by Russian drones in the southern Odesa region that hit a Danube port on the border with Nato member Romania.
Since pulling out of a key deal that allowed the safe passage of ships carrying grain, Russia has launched repeated attacks on targets in the Black Sea and on the Danube river.
This latest attack came on the eve of a summit in Russia between President Vladimir Putin and Turkish leader Recep Tayyip Erdogan, who hopes to revive the grain deal.
Ukraine said Russia had hit the Odesa region with a barrage of drones. Kyiv said some of the drones hit the Danube area, wounding at least two people in attacks on "civilian industrial" infrastructure.
Russia's army said it had aimed at "fuel storage" sites in the Ukrainian port of Reni, which lies on the Danube river that separates Ukraine from Romania.
Mr Zelenskyy also said he reached a "very important agreement on training our pilots in France" in a conversation with President Emmanuel Macron.
"Our coalition of modern fighters is becoming stronger," he said.
Western military aid has played a crucial role in the war, as Ukraine first forced back Russian troops around the capital Kyiv before launching counter-offensives in the north-east and south.
Its troops are now fighting through heavily mined areas and Russian defensive lines to try to recapture territory in the south-east and east.