Boris Johnson was spotted in the Ukrainian town of Bucha on Sunday morning chatting to residents as he toured the area.
The British MP and former prime minister was seen greeting a crowd before getting into a car, guarded by soldiers.
The Conservative politician's visit comes as he faces calls for an ethics investigation following reports he received help in the form of a loan from the man he later recommended to head the BBC.
Rishi Sunak is “supportive” of Mr Johnson’s visit to Ukraine, No 10 Downing Street said.
The Prime Minister’s press secretary said he was “always supportive of all colleagues showing that the UK is behind Ukraine and will continue to support them”.
Mr Johnson visited Ukraine three times during his tenure in No 10. His latest excursion to Ukraine is his first since stepping down as prime minister last September.
In a video released by the President’s office in Kyiv, Mr Johnson was seen approaching President Volodymyr Zelenskyy and a group of officials in Kyiv.
Mr Johnson could be heard saying that they “shouldn’t have bothered” with the formal greeting. “I’m too honoured,” he added.
“You do me too much honour, Mr President,” he told Mr Zelenskyy, who replied: “Good to see you Boris, welcome.”
Mr Johnson told Mr Zelenskyy that his entourage had driven past Hostomel, a city north-west of Kyiv, where he had witnessed the effects of “appalling” Russian attacks.
“And it’s still going on every day,” added Mr Johnson. “I will do whatever I can, I really will, I really will.”
Mr Johnson said his latest trip came after he was invited by Mr Zelenskyy.
“It is a privilege to visit Ukraine at the invitation of President Zelenskyy,” he said.
“The suffering of the people of Ukraine has gone on for too long.
“The only way to end this war is for Ukraine to win — and to win as fast as possible. This is the moment to double down and to give the Ukrainians all the tools they need to finish the job.”
Mr Johnson said the sooner Russian President Vladimir Putin's war failed, “the better for Ukraine and for the whole world”.
He took part in a wreath-laying ceremony in Bucha, where mass graves were uncovered last year following the Russians' withdrawal.
Mr Johnson was pictured alongside Orthodox priest Andrew placing flowers in tribute to those killed in the area, during a visit to a local church.
Mr Johnson's plans to travel to Ukraine were first reported in January and sparked criticism from MPs who warned he could risk undermining Mr Sunak's authority.
Tobias Ellwood, chairman of the Defence Select Committee in the UK Parliament, told The Guardian at the time that Mr Johnson should “not interfere with the messaging or the official lines of communication” between London and Kyiv.
Earlier this week, Mr Johnson spoke in a panel discussion on Ukraine at the World Economic Forum in Davos, where he was introduced as a “legendary figure” in Ukraine.
