KYIV—Ukrainian President Volodymyr Zelensky ousted his popular defense minister, a digital guru who drew support from Ukrainians and Western allies for his efforts to promote modern technologies to outsmart Russia and fight corruption.Mykhailo Fedorov at NATO headquarters in Brussels in June.Mykhailo Fedorov, a 35-year-old who served barely six months in the role, posted a valedictory message on social media late Wednesday after days of rumors of his ouster. He listed 22 achievements, including getting Elon Musk’s Starlink internet service disabled for Russians and introducing tenders for the purchases of military equipment from artillery shells to drones and pickup trucks.His dismissal provoked criticism among Ukrainians who credit Fedorov, a former digital innovation minister, for his efforts to modernize Ukraine’s armed forces. Plans for protests against the decision zipped across social media late Wednesday as Russia launched a wave of aerial strikes against Ukraine.Zelensky didn’t immediately comment on the removal of Fedorov, whose dynamism and prominence have led some to see him as a potential future rival to the president. The government collapsed earlier this week after Prime Minister Yulia Svyrydenko resigned following a meeting with Zelensky on Sunday.Ukraine has turned the tide on the front lines in recent months, largely halting Russian advances with the help of drone strikes that are sapping supplies of men and equipment to Russia’s front lines.“The season of shooting oneself in the foot has opened,” Kostyantyn Batozsky, a political analyst, wrote on social media.Zelensky on Wednesday endorsed the former head of Ukraine’s state natural gas company, Serhiy Koretskiy, as prime minister. The president called for unity between the army and the defense ministry, hinting at the discord between Fedorov and Ukraine’s top general, Gen. Oleksandr Syrskiy, over the prosecution of the war.“Unity is our great strength, and we must present a united front in the Ministry of Defense and our army,” Zelensky said.In his message Wednesday, Fedorov highlighted his efforts to encourage innovation to give Ukraine’s military the tools it needs to counter its much larger enemy.Serhiy Sternenko, an adviser to the defense ministry who has raised millions to purchase drones for Ukraine’s military, suggested dirt was being lobbed at Fedorov by corrupt factions enraged by his overhauls of procurement.“You can’t even imagine what streams of funding have been cut off,” Sternenko wrote on social media this week. On Wednesday, he said he had left his role as adviser.The question now is whether Ukrainians will muster sufficient protests, and the West will put enough pressure on Zelensky to change his mind. Last year, Zelensky reversed a proposal to strip Ukraine’s independent anti-corruption police of powers following street demonstrations and diplomatic interventions.Meanwhile, lawmakers said Zelensky would propose Interior Minister Ihor Klymenko as a replacement for Fedorov.Write to Anastasiia Malenko at anastasiia.malenko@wsj.com