Cowan was selected by Gov. Deval Patrick (D-Mass.) to temporarily fill Kerry’s former seat. A special election will be held in June to decide who will serve Massachusetts on a longer-term basis.
Kerry, who attended the swearing in, left the Senate to serve in President Obama’s cabinet as secretary of State. The Senate overwhelmingly confirmed Kerry for the position last week.
Members of the chamber applauded as Cowan signed the oath of office. Sen. Tim Scott (R-S.C.), the only other African American currently serving in the Senate, was one of the first people to shake Cowan's hand after he was sworn in.