Newly-elected senators included Tennessee Republican Bill Hagerty, Colorado Democrat John Hickenlooper, New Mexico Democrat Ben Ray Lujan, Wyoming Republican Cynthia Lummis, Kansas Republican Roger Marshall, Alabama Republican Tommy Tuberville and Arizona Democrat Mark Kelly.
Senate Majority Leader Mitch McConnell said the new Senate was convening at a "challenging time."
Nancy Pelosi was narrowly re-elected speaker of the U.S. House of Representatives on the same day, as a new Congress took office amid political uncertainty, with Senate control undecided and a Republican fight looming over presidential election results.
The House voted 216-209 to reinstate Pelosi, after Democrats lost 11 seats in the November elections to command a narrower 222-212 majority. Five Democrats chose not to support her - two voted for Democratic lawmakers who were not running, while three others simply voted "present."
"As we are sworn in today, we accept a responsibility as daunting and demanding as any that previous generations of leadership have faced. We begin the new Congress during a time of extraordinary difficulty," Pelosi said in a floor speech that noted the deaths of more than 350,000 Americans from the COVID-19 pandemic.
"Our most urgent priority will continue to be defeating the coronavirus. And defeat it, we will," she added, pledging that further aid would follow the latest $892 billion package that Congress passed in December.
The Senate remains Republican run ahead of twin elections in Georgia on Tuesday, giving its members a platform to again air President Donald Trump's baseless claims that his loss to Democratic President-elect Joe Biden was the result of fraud.
Multiple state and federal reviews have found no evidence of the sort of widespread fraud Trump claims, but Republican senators and House members plan to challenge the election result when Congress certifies it on Wednesday.
MIKE PENCE: So help you God.
- I do.
MIKE PENCE: Congratulations, Senator.
MITCH MCCONNELL: To say the 117th Congress convenes at a challenging time would indeed be an understatement. From political division, to a deadly pandemic, to adversaries around the world, the hurdles before us are many, and they are serious.
- The Honorable Nancy Pelosi of the state of California, having received a majority of the votes cast, is duly elected Speaker of the House of Representatives for the 117th Congress.
NANCY PELOSI: We begin this new Congress during a time of extraordinary difficulty. Each of our communities has been drastically, drastically affected by the pandemic and its economic crisis. 350,000 tragic deaths. We sadly carry them in our hearts.
It gives me great pride to serve as Speaker of the most diverse House of Representatives in the history of our country, with a record shattering 122 women--
122 women. You can applaud that. 122 women.
Our most urgent priority will continue to be defeating the coronavirus. And defeat it we will.
And defeat it we will.