Prime Minister Shinzo Abe announced the figure as he and fellow world leaders, including US President Barack Obama, cancelled an upcoming G8 meeting in Sochi, and said it would be replaced by a G7 event that did not involve Moscow.
Chief Cabinet Secretary Yoshihide Suga told reporters in Tokyo that Kiev needed help at a time of huge strain on the country's finances.
"It is extremely important that each country in the international community gives support so that Ukraine, facing a severe economic situation amid political confusion, will be able to restore economic stability," he said.
"Against that background, the prime minister announced that Japan will provide economic assistance of up to 150 billion yen ($1.5 billion) on condition that the Ukraine government will reach an agreement with the (International Monetary Fund) on economic reforms.
"Of the sum, 110 billion yen will be (low-interest) yen loans."
At the meeting in The Hague, the G7 also threatened tougher sanctions against Russia for its absorption of Crimea, which has plunged relations between the West and Moscow to their lowest point since the Cold War.
The gathering came as Ukraine ordered its outnumbered troops to withdraw from Crimea as yet another of its bases was stormed.
Earlier, the White House had said it was "very concerned" by a build-up of Russian troops on the border.
Tokyo has fallen into line with Washington and its allies in tightening the screws on Moscow, despite the differing strategic priorities for a nation entirely dependent on imports for its energy, with Russia a key global supplier of gas.
Abe has held multiple summits with Russian President Vladimir Putin since coming to office in late 2012 and was one of the few pro-Western leaders who attended the opening of the Winter Olympics in Sochi. Others stayed away to register disquiet over Moscow's anti-gay laws.
The Japanese leader has been pushing to expand the two countries' economic ties and resolve a decades-old territorial conflict at a time that Tokyo is embroiled in separate disputes with China and South Korea.
However, isolating Putin over Crimea threatens to derail progress towards resolving the issue, which has prevented Tokyo and Moscow signing a formal treaty ending World War II hostilities.