Zelenskiy arrived to shouts of support from people who had gathered outside of 10 Downing Street, where Starmer gave him a hug and ushered him inside.
"And as you heard from the cheers on the street outside, you have full backing across the United Kingdom," Starmer told the leader of the war-torn country.
"We stand with you, with Ukraine, for as long as it may take."
Zelenskiy thanked him and the people of the UK for their support and friendship.
The meeting comes a day after an extraordinary public spat when Trump and Vice President JD Vance criticised Zelenskiy in the Oval Office on live television for not being grateful enough for US support.
Zelenskiy had been poised to sign a deal to give the US access to mineral riches as Trump pressures Ukraine to reach a deal to end the war with Russia.
But he left Washington DC without signing anything.
Zelenskiy had been scheduled to meet with Starmer on Sunday before a summit with other European leaders to discuss Ukraine and shoring up defences across the continent.
But the timetable for their bilateral meeting was apparently sped up in the aftermath of the US visit.
The Ukrainian leader's plane, emblazoned with the country's flag, landed at London's Stansted Airport on Saturday morning and was met by a convoy of cars.
Zelenskiy struck a conciliatory tone in a series of posts on X after arriving in the UK.
Ukraine was "very grateful to the United States for all the support," Zelenskiy said.
"I'm thankful to President Trump, Congress for their bipartisan support, and American people," he added.
"It's crucial for us to have President Trump's support. He wants to end the war, but no one wants peace more than we do.
"We are the ones living this war in Ukraine.
"It's a fight for our freedom, for our very survival."
The verbal stoush at the White House was met with a volley of supportive messages from European leaders who are set to gather in London on Sunday.
The summit on Sunday in London will include leaders from France, Germany, Denmark, Italy, Netherlands, Norway, Poland, Spain, Turkey, Finland, Sweden, the Czech Republic and Romania, as well as the NATO secretary-general and the presidents of the European Commission and European Council.
Zelenskiy will meet with King Charles on Sunday before the summit that is being held at Lancaster House, a 200-year-old mansion near Buckingham Palace.
Three people were killed in a Ukrainian attack on a Russian-occupied part of Kherson region in southern Ukraine, Russian-installed governor Vladimir Saldo said on Saturday.
Two of them were killed on a road between the villages of Nova Maiachka and Obryvka, while the third was killed in the town of Oleshky, Saldo said via his channel on Telegram.
Five other people were injured of varying degrees of severity, Saldo said.
A Russian missile struck port facilities in Ukraine's southern city of Odesa on Saturday, injuring two port workers and damaging infrastructure and a vessel, regional governor Oleh Kiper said.
Kiper said on Telegram the strike damaged a Panamanian-flagged vessel belonging to a European company.Â
He said emergency crews were at the site and medics were treating the two injured men.
with PA and AP
