During a surprise visit to the country's war-torn capital on Thursday, Harry delivered a lengthy, impassioned speech at the Kyiv Security Forum, saying he was "not here as a politician" but as "a soldier who understands service" and as a humanitarian.
In a rare input on global matters, the King's youngest son also turned his attention to the US - although he did not mention President Donald Trump by name - urging the "American leadership" to "honour its international treaty obligations" in its "enduring role in global security".
Addressing Putin, Harry called on the Russian leader to "prevent further suffering" for both Ukrainians and Russians and choose a different path.
"President Putin, no nation benefits from the continued loss of life we are witnessing," Harry said.
"There is still a moment - now - to stop this war, to prevent further suffering for Ukrainians and Russians alike, and to choose a different course."
He condemned the "systematic war crimes", and the "chilling" treatment of Ukraine's children.
Harry suggested the conflict is "not simply a war about territory", but one about "values" and "sovereignty".
He praised the Ukrainian people's endurance, saying "this is what leadership looks like in the modern world", and spoke of the "staggering" human toll of the war.
The duke, who served on two frontline tours to Afghanistan, said: "I am not here as a politician.
"I am here as a soldier who understands service, as a humanitarian who has seen the human cost of conflict, and as a friend of Ukraine who believes the world must not grow used to this war or numb to its consequences."
Harry was filmed by ITV News arriving at Kyiv railway station on Thursday morning, stepping off an overnight train from Poland and greeting and hugging people on the platform.
"It's good to be back in Ukraine, a country bravely and successfully defending Europe's eastern flank," Harry also told The Sun newspaper.
His speech comes just four days before his father the King begins a major state visit to America to see Trump, against the backdrop of trans-Atlantic tensions over the Iran conflict.
Harry's unannounced visit - his third trip to Ukraine since the war began in 2022 - comes days after he finished a tour to Australia with his wife, the Duchess of Sussex.
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