No 10 said the visit was a "show of solidarity" with the Ukrainian people.
Following the talks, Downing Street said the UK would send 120 armoured vehicles and anti-ship missile systems to support Ukraine.
It comes the day after the UK announced £100m of weapons for the country after the bombing of refugees at a railway station.
A No 10 spokesman said: "The prime minister has travelled to Ukraine to meet President Zelensky in person, in a show of solidarity with the Ukrainian people."
Mr Johnson's visit to Kyiv was not announced in advance and the first indication he was in the city came when a photograph of his meeting with President Zelensky was put on Twitter by the Ukrainian embassy in London.
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