Yevhenii Khmara, a former top special forces operative, takes up the critical defence portfolio amid fresh political turmoil for the president, who had faced weeks of protests over his dismissal of 35-year-old reformer Mykhailo Fedorov.
Khmara's appointment also took place just hours after anti-corruption authorities announced charges against a deputy chief of Zelenskiy's office as part of an alleged $US3 million ($A4.2 million) money-laundering scheme.
Fedorov's removal as defence minister in July brought thousands of demonstrators onto the streets in rare wartime protests over a surprise shake-up seen as poorly communicated to the public at a critical time in the war with Russia.
Fedorov's bold call for elections in the fifth year of fighting drew accusations of populism in a country where a majority of people still oppose a wartime vote, even though many argue that Ukraine needs fresh faces in power.
"He chose to move on the populist trajectory, underlining the risks and challenges for democratic process of election," said Lesia Bidochko, a policy fellow at the European Policy Institute in Kyiv.
Zelenskiy has yet to respond directly to Fedorov's call, but the president's communications adviser Dmytro Lytvyn told reporters that: "Russia's daily drone and missile strikes are the exhaustive answer to this question".
In his statement late on Tuesday, Fedorov said the country faced a crisis of governance and that Ukraine's democratic process "cannot be held hostage by Russia".
Wartime elections are prohibited under law.
An August 5 poll by the Kyiv International Institute of Sociology showed 57 per cent of Ukrainians believe they should only take place after the war, given that hundreds of thousands of troops are on the front line and Russian missiles and drones are raining down on Ukrainian cities, towns and villages.
"We must find a legal, safe and realistic mechanism that will allow Ukraine to renew its full democratic process even in the conditions of a long war," said Fedorov.
He did not say whether he would compete in any election, but another recent poll indicated the tech-savvy Fedorov - Zelenskiy's longest-standing ally - would comfortably beat his former boss in a runoff vote.
Bidochko said there was still no realistic procedure for organising a vote, with millions of Ukrainians living as refugees overseas, displaced within the country, and serving in the military.
"Currently, it looks like an elegant way of occupying the opposition space without taking responsibility for the mechanics," she said.
Opinion on the streets and online was mixed on Wednesday.
