Walsh, 53, replaces former Sen. Max Baucus, also a Democrat, who was confirmed last week, in a 96-0 Senate vote, as U.S. ambassador to China.
Baucus revealed plans to retire in 2014 from the Senate after 30 years, and Walsh had already announced his candidacy for Montana's open seat. Walsh's appointment to the Senate is seen as providing an advantage to Montana Democrts in the November election.
Democrats hope to keep the seat in what is expected to be a competitive race against Rep. Steve Daines, R-Mont., the Washington newspaper The Hill reported Tuesday.
2014: The Year in Music [PHOTOS]