Paul Kirk said it would be an honour to serve as senator
The governor of Massachusetts has named Paul Kirk as a temporary replacement to fill the Senate seat left vacant by the death of Edward Kennedy.
Mr Kirk, 71, is a former Democratic Party chairman and worked for Sen Kennedy for eight years.
The move means that Democrats in the US Senate have regained the majority needed to push through President Obama's healthcare reform agenda.
They fell one seat short through the death of Sen Kennedy.
The Massachusetts state senate voted on Wednesday to allow the state governor to appoint a temporary replacement senator.
Without the change, Sen Kennedy's seat would have stayed vacant until January.
The rule change was one of Edward Kennedy's last wishes.
The late senator feared that if his fellow Democrats did not have a full complement of senators, passing a healthcare reform bill would be more difficult.
Mr Kennedy campaigned throughout his career for universal healthcare, and did not want his death to jeopardise prospects for reform.
His widow Vicki Kennedy and son Edward Kennedy Jr had urged Governor Deval Patrick to choose Paul Kirk.
"He (Sen Kennedy) often said that representing the people of Massachusetts in the Senate of the United States was the highest honour that he could possible imagine, and it is certainly nothing that I imagined, but it will be my highest honour, as well," said Mr Kirk.
He is expected to be sworn in on Friday and will serve until 19 January.