The outgoing Labour leader on Thursday morning arrived in the war-torn capital Kyiv, where he will meet President Volodymyr Zelenskiy.
He is expected to tell the Ukrainian President that the UK's commitment to backing Ukraine will continue after he is replaced by Andy Burnham, who is set to be the fifth prime minister since Russia's invasion of the country in 2022.
Burnham will become Labour leader on Friday before taking over as prime minister on Monday.
Starmer arrived at a Kyiv train station hours after strikes were reported in the Ukrainian capital, with the mayor Vitali Klitschko saying that two people had died and six others, including a 16-year-old teenager, had been injured.
Before his trip, Starmer said: "Throughout this conflict, I have seen the incredible fortitude of the Ukrainian people and the iron will of a nation that refuses to be cowed.
"Their stand has not only defended its own freedom, it has preserved the security of Europe.
"When I became prime minister, I knew the UK must not simply stand with Ukraine in the moment, but help build the foundations of its long-term security and success.
"That's why we put the UK at the centre of a stronger Europe - investing more money into defence, leading the way in the war fighting technologies of the future and doing everything we can to put Ukraine in the strongest possible position.
"I am so proud of what Britain has contributed. That work will continue, and our cast-iron support for Ukraine will always endure."
Starmer will also hear how allies can further meet the needs of Ukraine.
The first of 150 British-made, artillery barrels are being provided to Ukraine to strengthen their defence under a contract with BAE Systems.
The prime minister's trip comes days after Ukrainian air defences intercepted five ballistic missiles launched by Russia in a raft of night attacks, though other missiles and drones got through and hit Kyiv.
In the capital, the attack caused fires at two warehouses, while a school was also damaged, the mayor Vitali Klitschko said.
The Russian defence ministry said the attack targeted military manufacturing facilities in the Ukrainian capital that produce long-range missiles and drones.
Ukraine's air force said one ballistic missile and 25 drones struck 17 locations, while falling debris was reported in 10 locations.
The prime minister is spending his final days in office highlighting what he sees as his legacy, including support for Ukraine.
He began the week in Paris where he attended a meeting of the "coalition of the willing" group.
On Tuesday morning, he was with Emmanuel Macron and other leaders at the Bastille Day celebrations in the French capital in a sign of the closer relationship he has tried to forge with European neighbours as part of his post-Brexit reset.
Macron awarded Starmer the Legion d'honneur in recognition of his support for Ukraine and European relations.
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