Vice President Joe Biden administered the oath of office in a chamber peopled by a dozen Democratic senators and a handful of Republicans.
As he walked up the center aisle to meet Biden, Schatz, 40, took Akaka's hand and helped the frail Democratic senator, who is retiring, stay at his side.
Schatz had flown to Washington hours earlier on Air Force One with President Barack Obama. Democratic Gov. Neil Abercrombie named Schatz, who had been lieutenant governor, to succeed Inouye. Inouye died last week of respiratory complications at the age of 88.
The selection went against the dying wishes of Inouye, who is revered in Hawaii politics. He had wanted Rep. Colleen Hanabusa to succeed him.
Schatz is a former state representative and onetime chairman of the state Democratic Party who ran Obama's 2008 campaign in Hawaii.
He said his top priorities in the Senate would be addressing global climate change, preserving federal funds used in Hawaii for things like defense spending and transportation and getting federal recognition for Native Hawaiians to form their own government, similar to many Indian tribes.
Schatz told reporters in Washington that he slept for most of the long flight, but he also spoke briefly with Obama.
"We're anxious to get to work" to try to avert the fiscal cliff, Schatz said, referring to a package of large tax hikes and spending cuts that will take effect in the new year unless Congress intervenes.
Schatz said it was "difficult to understand why we've inflicted this on ourselves." He said the only thing worse than some of the tax hikes and spending cuts proposed to avert the cliff "is not fixing it."
"I'll be looking forward to supporting the administration's priorities," he added.
Schatz, wearing on overcoat bought specifically for the trip, said Abercrombie informed him of his appointment shortly before it was announced publicly. He said he didn't know he would fly with the president to Washington. The rest of his family, including his parents, wife and children, flew commercially.
Schatz beat out Hanabusa and Esther Kiaaina, a deputy director in the state Land and Natural Resources Department. The three candidates were selected by state Democrats Wednesday morning from a field of 14. The candidates briefly made their cases before the state party's central committee.
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Obama returned home early from his Hawaii Christmas vacation as Congress considered what to do about the so-called fiscal cliff.
Schatz will become Hawaii's senior senator in the new Congress, which convenes in a few days. Akaka is retiring after 22 years, and Democratic Rep. Mazie Hirono was elected in November to succeed him.
Inouye was by far Hawaii's most influential politician and one of the most respected lawmakers in Washington after serving five decades in the Senate. He sent Abercrombie a hand-signed letter dated the day he died, saying he would like Hanabusa to succeed him, calling it his "last wish."
Four days after eulogizing Inouye in the courtyard of the Hawaii Capitol, Abercrombie said he had to consider more than just Inouye's wishes in filling his seat.
"Of course Sen. Inouye's views and his wishes were taken into account fully, but the charge of the central committee, and by extension then myself as governor, was to act in the best interests of the party ... the state and the nation," Abercrombie said.
Under state law, the successor had to come from the same party as the prior incumbent. An Abercrombie spokeswoman said the governor did not feel any political pressure from within his party to make the choice he made.
Press; By CHARLES BABINGTON]
Associated Press writers
Matthew Daly and Becky Bohrer in Honolulu contributed to this
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