Japan is one of Washington's closest allies, but Trump alarmed Tokyo during his presidential campaign by talking about pulling thousands of US troops from East Asia.
Japan's Prime Minister Shinzo Abe described Donald Trump as a "trustworthy leader" after meeting the US president-elect on Thursday to get clarity on statements Trump had made over the alliance's character.
Trump had fanned worries in Tokyo and beyond with comments on the possibility of Japan acquiring nuclear arms and demands that allies pay more for keeping US forces on their soil or face their possible withdrawal.
"The talks made me feel sure that we can build a relationship of trust," Abe told reporters after the hastily arranged 90-minute meeting at Trump Tower in Manhattan.
But he would not disclose specifics because the conversation was unofficial.
Trump, in a brief entry on his Facebook page accompanied by a photo of the two men, said, "It was a pleasure to have Prime Minister Shinzo Abe stop by my home and begin a great friendship."
Japan's leadership has been nervous about the future of an alliance that is core to Tokyo's diplomacy and security since Trump raised concerns about its prospects with his statements during the US election cycle.
Trump had also expressed his opposition to the US-led 12-nation Trans-Pacific Partnership (TPP) free trade pact.
Abe had worked closely with President Barack Obama on the TPP trade pact, which was part of Obama's push to counter the rising strength of China and a key pillar of Abe's economic reforms.
"Alliances cannot function without trust. I am now confident that President-elect Trump is a trustworthy leader," said Abe, describing the talks as "candid" and held in a "warm atmosphere."
Abe said he had agreed to meet again with Trump "at a convenient time to cover a wider area in greater depth."
It was unclear if that would happen before Trump's inauguration on January 20. Trump official Kellyanne Conway told CBS earlier on Thursday that "any deeper conversations about policy and the relationship between Japan and the United States will have to wait until after the inauguration."
Also at the meeting with Abe was Trump's model-turned-business executive daughter Ivanka and her husband, real estate developer and publisher Jared Kushner.
The presence of the couple, who have emerged as key advisors, underscores the family's influence as the president-elect readies to take power.
Abe gave Trump a golf driver and received golf-wear in return, Japanese officials noted.