William "Mo" Cowan of Massachusetts is the Senate's newest member.
The 43-year-old Democrat took his seat Thursday after a swearing-in ceremony led by Vice President Biden. Cowan succeeds Democrat John Kerry, who just took over as secretary of state.
Cowan was picked by Democratic Gov. Deval Patrick to serve as interim senator until after a special election June 25. Cowan had stepped down as Patrick's top aide last December.
Cowan is the state's second African-American senator. Republican Edward Brooke served from 1967 to 1979.
With Cowan and Sen. Tim Scott, R-S.C., there are now two African-American senators serving together for the first time. Scott was also a gubernatorial appointee to his seat.
Cowan was a partner in a Boston law firm before going to work for Patrick.