State Senator James Timilty announced Friday that he would leave the Senate to take the job of Norfolk County treasurer, overseeing the pension and retirements of the nearly 300-employee county government.

The Walpole Democrat’s departure from the Senate will create a second vacancy in the Senate, after the death of Senator Kenneth Donnelly this month, and trigger another special election.

“While I look forward to this new opportunity, I want to thank the tremendous people I am leaving in the Senate,’’ Timilty wrote in a statement on Twitter on Friday afternoon.

Timilty said, “I will truly miss being the Senator from the Bristol & Norfolk.’’

Timilty was first elected to the Senate in 2004.

Frank Hegarty, the county director, said Norfolk County Treasurer Joseph Connolly submitted his resignation on Wednesday, effective April 28, and the county commissioners unanimously decided to select Timilty as his replacement at their regular meeting on Wednesday.