Vice President Joe Biden swore in Booker in the Old Senate Chamber Thursday afternoon.
The former mayor of Newark, New Jersey, becomes the second African American in the Senate. The other is Republican Tim Scott of South Carolina.
Booker took the oath on his own Bible. The Democrat was surrounded by family, including his mother and brother. His father died Oct. 10.
Booker and his mother met with Majority Leader Harry Reid Thursday morning. In the afternoon, he'll meet with President Barack Obama in the Oval Office.
Booker is serving out the remaining term of the late Democratic Sen. Frank Lautenberg, who died in June.
The Democratic majority stands at 55 senators.
