Kirsten Gillibrand, a little-known, pro-gun Democrat from upstate New York, has been sworn into the U.S., Senate, succeeding Hillary Rodham Clinton.
Vice President Joe Biden administered the oath on Tuesday in the U.S. Capitol building.
New York Gov. David Paterson appointed the two-term congresswoman to the Senate seat last Friday. She was named to the post after Caroline Kennedy, daughter of President John F. Kennedy and the presumed front-runner, withdrew from contention because of personal reasons.
Gillibrand fills a seat vacated when Clinton became secretary of state.
At 42, Gillibrand is the youngest U.S. senator. She is the 17th woman in the chamber.
Gillibrand represented a rural Republican district. She is a lawyer by trade and the mother of two young boys who were present for the ceremony along with their father.
After she was sworn in, she told reporters that becoming a senator was something she never could have imagined.
"I feel incredibly grateful that the governor placed his trust in me," Gillibrand said. "To have the opportunity to serve this entire state at a time of great economic crisis is an extraordinary responsibility and I take that responsibility very seriously."
Her appointment lasts until 2010, when a special election will be held to fill the final two years of Clinton's term.