Vice President Joe Biden administered the oath on Tuesday in the U.S. Capitol.
Gov. David Paterson appointed the second-term congresswoman to the Senate seat last Friday. She was named to the post after Caroline Kennedy, the presumed front-runner, withdrew from contention because of personal reasons.
Gillibrand fills a seat vacated when Sen. Hillary Rodham Clinton became secretary of state.
At 42, Gillibrand is the youngest U.S. senator. She's the 17th woman in the chamber.
Gillibrand represented a rural Republican district. She's a lawyer by trade and the mother of two young boys.