Massachusetts Governor Deval Patrick on Wednesday appointed his former chief of staff to temporarily fill the Senate seat held by John Kerry, who has been confirmed as incoming secretary of state.
Patrick said he was "proud and delighted" to name William "Mo" Cowan to serve as interim senator until a special election is held on June 25.
"I pledge to you and the people of Massachusetts that during this interim period I will work as hard as humanly possible to perform my duties with the needs and aspirations of our state's citizens foremost in mind," Cowan said in a statement.
Cowan, 43, will become the second African-American member of the US Senate, after South Carolina's governor appointed congressman and rising Republican star Tim Scott to replace conservative senator Jim DeMint, who resigned to run the Heritage Foundation think tank.
In picking a relative political unknown, Patrick passed over other potential appointees including long-serving congressman Barney Frank, who retired early this month, and Vicki Kennedy, the widow of former US senator Ted Kennedy.
The departure of Kerry, whose confirmation breezed through the Senate on Tuesday, sets off a scramble for a seat in a liberal state where Democrat Elizabeth Warren defeated incumbent Senate Republican Scott Brown in November.
Democratic congressman Edward Markey has already announced his intention to run for the seat, while a Republican primary could see the defeated Brown attempt another bid for the Senate.