The Duke of Sussex arrived by train and said he wanted to do "everything possible" to help the recovery of injured military personnel.
Superhumans, which helps provide those injured with prosthetic limbs and rehabilitation, told the BBC that it invited Prince Harry to Ukraine.
There are tens of thousands of soldiers and civilians with amputations as a result of Russia's full-scale invasion of Ukraine – numbers vary, as Ukraine doesn't give precise statistics on military casualties.
The prince took part in a panel discussion at Kyiv's National Museum of the History of Ukraine in the Second World War - where he advised those leaving military service that there is "light at the end of the tunnel".
"You will feel lost at times, like you lack purpose," said Harry, who spent 10 years in the British army.
"Don't stay silent. Silence will hold you in the dark.
"Open up to your friends and family, because in doing so you give them permission to do the same."
Among the people Harry met during the trip was war veteran Vasyl Tamulis, who told the Reuters news agency: "My main goal was to get a photograph with him because not many people have a photograph with [a] prince."
"Being selected for Invictus Games unites people and motivates because it is a very difficult selection process," he added, referring to the international multi-sport competition Harry set up for injured and sick military service personnel - both serving and veterans.
The duke also met privately with Ukraine's Minister for Veteran Affairs Natalia Kalmykova and attended a fundraising lunch in support of the Superhumans Centre in Lviv.
Ahead of the trip, Prince Harry told the Guardian: "We cannot stop the war but what we can do is do everything we can to help the recovery process."
"We can continue to humanise the people involved in this war and what they are going through."
The paper reported that Harry was joined by a team from his Invictus Games Foundation, which he launched in 2014.
Ukraine was given special permission to compete in the games by President Zelensky in 2022, just months after the war began.
During the opening ceremony, the prince said the world was "united" with the country.
His visit to Kyiv came after the Sussex's charitable foundation Archewell said on Wednesday that it had donated $500,000 (£369,000) to projects supporting injured children from Ukraine and Gaza.