Mr Reznikov had led the ministry since before the start of Russia’s full-scale invasion in February 2022.
President Volodymyr Zelensky announced Mr Reznikov’s dismissal on Sunday, saying it was time for “new approaches” in the defence ministry.
Rustem Umerov, who runs Ukraine’s State Property Fund, has been nominated as Mr Reznikov’s successor.
In a post on X, formerly Twitter, Mr Reznikov confirmed that he had submitted his resignation letter to the country’s parliament.
Ukrainian media has speculated that he will become Kyiv’s new ambassador in London, where he has developed good relations with senior politicians.
The 57-year-old has become a well-known figure since the beginning of the war in Ukraine. Internationally recognised, he has regularly attended meetings with Ukraine’s western allies and played a key role in lobbying for additional military equipment.
But his dismissal has been anticipated for some time. Last week, Mr Reznikov told reporters he was exploring other positions with the Ukrainian president.
According to local media, the former defence minister said that if Mr Zelensky offered the opportunity for him to work on another project he would probably agree.
Ukrainian defence advisor Yuriy Sak told the BBC that Mr Reznikov spearheaded the transformation of the ministry, laying the groundwork for future NATO membership.
“His legacy is that he has convinced ministers of defence around the world that the impossible is possible,” he said in reference to Mr Reznikov’s successful lobbying of foreign governments for arms.
But experts have observed that the cabinet reshuffle is unlikely to lead to any major change in Ukraine’s battlefield strategy, with Gen Valery Zaluzhny – the commander of Ukraine’s armed forces – overseeing the campaign.
Mr Reznikov’s dismissal comes amid a wider anti-corruption drive in Mr Zelensky’s administration – which has been seen as essential to Ukraine’s ambitions to join Western institutions like the EU.
According to the Transparency International’s Corruption Perceptions Index, Ukraine ranks 116th out of 180, but efforts in recent years have seen its position improve significantly.
While Mr Reznikov is not personally accused of corruption, there have been a number of scandals at the ministry of defence involving the procurement of goods and equipment for the army at inflated prices.
Earlier this year Mr Reznikov’s deputy, Vyacheslav Shapovalov, resigned in the wake of the scandal. It was widely reported at the time that Mr Reznikov barely held on to his own post.
At the time, he said the stress he had endured was “hard to measure precisely”, adding that his “conscience is absolutely clear”.
The defence ministry has also been rocked by several recent arrests at regional recruitment offices, where officers have been accused of taking bribes to allow men to avoid Ukraine’s military draft.
On Friday, the US National Security Adviser Jake Sullivan met with senior Ukrainian anti-corruption officials and urged them to continue prosecuting anti-graft cases “no matter where they lead”.
Mr Umerov, who was nominated by the president, represented Ukraine in peace talks at the beginning of Russia’s full-scale invasion.
The ex-MP allegedly suffered symptoms of suspected poisoning during peace negotiations in March 2022 alongside Russian billionaire Roman Abramovich – who was also part of the negotiating party. In a statement posted to Facebook he later denied the reports, urging people not to trust “unverified information”.
Speaking to the BBC at the time, he said it took courage to find solutions but he was determined “to find [a] political and diplomatic resolution to this brutal invasion”.
A member of the Crimean Tatar community, he has become a key member of Mr Zelensky’s international outreach efforts, focussing on fostering ties in the Islamic world.
Mr Reznikov’s dismissal comes as Ukraine wages a slow and bloody counter-offensive after securing more advanced weapons from Western allies.
Progress on the frontline has been slow but top Ukrainian generals said on Sunday that their forces have broken through a key line of Russian defences in the south of the country.
Meanwhile, Russia reported several attempted drone attacks on its territory overnight.
The defence ministry said it shot down two drones over the Kursk region, which borders Ukraine, early on Monday.
The region’s governor Roman Stravoit also reported on Sunday that debris from a destroyed drone had caused a fire at a non-residential building in the city of Kurchatov.
Elsewhere, Russia launched its own 3.5-hour overnight assault on the south of Ukraine’s Odesa region, with the governor reporting that 17 drones were downed.
“Unfortunately, there are also hits,” added Oleh Kiper, who said there had been damage to “several settlements” in the region of Izmail. But he said there were no casualties or injuries.
Izmail is one of Ukraine’s two major grain-exporting ports on the Danube River in the Odesa region.
The Danube ports have become Ukraine’s major exporting route since the collapse of the Black Sea grain deal in July. Moscow has launched frequent attacks on the Danube since withdrawing from the deal.