Kennedy, a close and early supporter of President Barack Obama, appeared before a Senate panel as she sought confirmation for her most public role since she was a bouncy girl in the White House from 1961-63.
"I can think of no country in which I would rather serve than Japan," said Kennedy, with two of her three children and other members of the political dynasty sitting behind her.
The 55-year-old said that she first visited Japan in 1978 with her uncle, late senator Ted Kennedy, and was "deeply affected by our visit to Hiroshima," which the United States obliterated in the world's first atomic bombing in 1945.
"These are areas I care deeply about and, if confirmed, I will work to further strengthen this critical partnership at a vital moment in its history," she said.
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The Senate appeared virtually certain to confirm Kennedy, meaning that she would head to Tokyo ahead of the 50th anniversary on November 22 of her father's assassination, when she was five days short of her sixth birthday.
"This appointment has a special significance as we commemorate the 50th anniversary of my father's presidency," she said.
Kennedy said that her father, who was seriously wounded by a Japanese destroyer in World War II, had hoped to pay the first US state visit to Tokyo. Gerald Ford eventually became the first US sitting US president to travel to Japan in 1974.
Senator Chuck Schumer, introducing Kennedy to the Senate Foreign Relations Committee, said he believed her father and other late family members were "looking down with pride" on her.