Announcing the visit on Twitter, 10 Downing Street shared an image of Johnson speaking to Zelensky outside the Mariinsky Palace in Kyiv and added Johnson's message was: "Ukraine can and will win this war."
Johnson has been one of the most vocal supporters of Ukraine as it tries to defend itself against Russia's unprovoked assault and the trip on Wednesday was his third visit to the Ukrainian capital since the war started in late February.
He became one of the first foreign leaders to make the precarious trip to the Ukrainian capital in late April, then returned on another surprise visit in June.
Johnson has forged a close relationship with Zelensky, who said he was sad to see the British PM leave office when the ruling Conservative Party forced him to resign in July.
On Wednesday, Zelensky presented Johnson with a parting gift, awarding him the Order of Freedom for "the work that Boris has been doing for our country and all of Europe."
The UK has poured more than £2.3 billion ($2.71 billion) into military and financial aid to Ukraine since the invasion began, according to a statement from Downing Street.
On Wednesday, the UK government announced it will provide Ukraine with mine-hunting vehicles, drones and loitering munitions worth £54 million ($63.5 million).