Brown's new Senate colleagues applauded after the ceremony on the floor of the Senate chamber conducted by Vice President Joe Biden, in his capacity as president of the Senate. Democratic Sen. John Kerry, also of Massachusetts, and Brown's wife, Gail Huff, accompanied Brown into the chamber.
At a news conference after the ceremony, Brown thanked Massachusetts voters who gave him a surprise victory in the January 19 special election in a state that had not elected a Republican senator since 1972.
"I can't promise I'll be right in every vote I make. I'm sure I'll make mistakes from time to time," Brown said. "But I will try to learn and grow and do the very best job I can on a day-to-day basis."
Updated: 6:20 p.m.