Japanese Prime Minister Fumio Kishida travelled to Kyiv for talks with Ukrainian President Volodymyr Zelenskyy on Tuesday.
The visit follows a trip to Moscow by Chinese President Xi Jinping to meet Russian counterpart Vladimir Putin on what was described as a visit of peace and friendship.
Japan's Foreign Ministry said Mr Kishida will “show respect to the courage and patience of the Ukrainian people who are standing up to defend their homeland” under Mr Zelenskyy's leadership.
Mr Kishida's trip to Ukraine began hours after a meeting with Indian Prime Minister Narendra Modi in New Delhi.
Japan’s public television NHK showed Mr Kishida aboard a train from Poland to Kyiv.
Mr Kishida will chair the G7 summit in Hiroshima in May. He said the summit should demonstrate a strong will to uphold international order and rule of law after Russia's invasion of Ukraine.
He is the only G7 leader who has not visited Ukraine. US President Joe Biden visited Kyiv last month, just before the first anniversary of Russia's invasion of Ukraine.
Japan has joined the US and European nations in sanctioning Russia following its invasion and providing humanitarian and economic support for Ukraine.
Mr Kishida will also hold talks with his Polish counterpart before returning to Japan on Thursday, the ministry said.
Mr Kishida is Japan's first postwar leader to enter a war zone.
In line with its pacifist principles, Japan’s support for Ukraine has also been limited to non-combat military equipment such as helmets, bulletproof vests and drones and humanitarian supplies including electrical generators.
Japan has contributed more than $7 billion to Ukraine and accepted more than 2,000 displaced Ukrainians and helped them with housing assistance and support for jobs and education, an unusual move for a country with a strict immigration policy.
