The British prime minister was pictured at a table opposite Mr Zelensky on Saturday, with the Union Jack and Ukrainian flag in the background.
It is believed to be the first time the pair have met in person since Russia invaded Ukraine a month and a half ago, starting a war reported to have killed hundreds of civilians and displaced millions more.
A Downing Street spokesperson confirmed the two had met in Kyiv on Saturday after an image of the leaders together was met with surprise on social media.
“The prime minister has travelled to Ukraine to meet President Zelenskyy in person, in a show of solidarity with the Ukrainian people,” the No 10 spokesperson said.
“They will discuss the UK’s long term support to Ukraine and the PM will set out a new package of financial and military aid.”
Andriy Sybiha, a Ukrainian presidential aide, said Mr Johnson had started his visit to Kyiv with a “tete-a-tete” meeting with Mr Zelensky.
Ukraine’s embassy in the UK shared an image of the two leaders meeting on Saturday, with the caption “surprise” and a winking face emoji.
Serhiy Leshchenko, an advisor to Mr Zelensky’s chief of staff told The Independent Mr Johnson’s visit to Kyiv was “an important show of solidarity” that not only sent a signal to global leaders to stand with Ukraine but also to Ukrainians to restart businesses, and cautiously try “a new normality with limitations”.
“We have to restart our economy because the frontline cannot survive if the economy doesn’t work. That is why it is also an important signal for Ukrainians that Boris Johnson is in Kyiv today,” he told The Independent.
He said that he hoped the visit would see the UK supply Ukrainians with more weapons, as they remained outgunned by Russia.
He said the president still hopes Europe would ultimately “close the skies” and impose a no-fly zone.
“We are very successful on the ground but Putin is bombing our cities, destroying our infrastructure and killing our people that is why we need anti aircraft weapons and why we we need more financial support,” he added.
He said he also hoped that the UK would invest in reconstruction of Ukraine’s levelled cities to allow the country to continue.
Oleg Nikolenko, a spokesperson for Ukraine’s foreign ministry, heralded Mr Johnson as one of the “most principled opponents of the Russian invasion” and added that the visit serves as a sign of the “ironclad solidarity and unwavering support” from the UK.
He said the two leaders agreed that sanctions should be increased until Mr Putin stops the hostilities and withdraws from Ukraine.
“We highly appreciate the announcement of new military and financial aid packages for Ukraine,” he added. “Ukraine requires more weapons, financial and humanitarian assistance to be able better defend itself.”
Following the talks between the two leaders, No 10 announced it would be sending 120 armoured vehicles and anti-ship missile systems to support Ukraine during a “crucial phase” of the war.
In a statement, Mr Johnson said it had been a “privilege” to travel to Ukraine and meet Mr Zelensky in person.
“Ukraine has defied the odds and pushed back Russian forces from the gates of Kyiv, achieving the greatest feat of arms of the 21st century,” he said.
The prime minister added: “I made clear today that the United Kingdom stands unwaveringly with them in this ongoing fight, and we are in it for the long run.
“We are stepping up our own military and economic support and convening a global alliance to bring this tragedy to an end, and ensure Ukraine survives and thrives as a free and sovereign nation.”
The announcement of more aid came the day after the UK announced a further £100m worth of UK military assistance for the Ukrainian forces, including anti-tank and anti-aircraft weaponry.
Ukraine had urged western allies to send more arms as they prepared for an offensive in the east, as Moscow pulled forces back from around Kyiv.
On Saturday, Ukrainian authorities told civilians to flee eastern areas. The day before, a train shelter where civilians were sheltering was bombed, killing at least 52 people.
The Tory party chair had previously said the UK prime minister was “desperate” to visit Ukraine to witness what was happening in the country under siege.
But a senior government official told The Independent last month such a trip would be “nonsensical” and his time would be “better spent addressing Home Office dysfunction” over the UK’s response to the refugee crisis sparked by the war.
Mr Johnson is believed to have last visited Ukraine and met Mr Zelensky at the start of February as the threat of a Russian invasion loomed.
The Independent has a proud history of campaigning for the rights of the most vulnerable, and we first ran our Refugees Welcome campaign during the war in Syria in 2015. Now, as we renew our campaign and launch this petition in the wake of the unfolding Ukrainian crisis, we are calling on the government to go further and faster to ensure help is delivered.