"While I began last week to formally step out of politics and it's very important that I respect the apolitical nature of the post I hope to soon occupy, as Massachusetts' senior senator today and as a colleague of Ed Markey's for 28 years, I'm excited to learn of and support his decision to run for the United States Senate," Kerry said in a statement.
Democratic Senatorial Campaign Committee Chairman Michael Bennet of Colorado also endorsed Markey, the Washington publication Roll Call reported.
"Ed Markey always remembers where he came from and will continue the hard work needed to turn our economy around," Bennet said. "He is exactly the kind of leader Massachusetts needs in the U.S. Senate."
Roll Call said the Boston Herald reported Sen. Edward M. Kennedy's widow, Vicki Kennedy, is endorsing Markey as well.
Markey, who is said to have experienced frustration being in the House minority party, says he will seek Kerry's seat if Kerry becomes secretary of state. President Obama has nominated Kerry for the Cabinet post.
"I have decided to run for the U.S. Senate because this fight is too important," Markey said in a statement to supporters. "There is so much at stake."
Markey, 66, the dean of the Massachusetts delegation, is the first prominent candidate to declare for Kerry's seat, which would be filled through a special election early next summer, probably in June.
The Massachusetts special election is likely to be closely watched and a potential harbinger for the 2014 mid-term elections, The Boston Globe said.
Kerry, who heads the Senate Foreign Relations Committee, is widely expected to be confirmed by the Senate to replace Secretary of State Hillary Clinton in Obama's second term.
Markey believes he would be able to do more in the Senate than in the House where the Republican Party is in control, a person familiar with Markey's decision told the Globe.
As a member of the House minority, even Markey's 36-year tenure is largely powerless, the Globe said. By contrast, a freshman Democratic senator in the Democrat-controlled Senate can push national platforms and exert great power, the newspaper said.
Democratic Massachusetts Gov. Deval Patrick is expected to appoint a temporary senator to fill Kerry's seat if Kerry is confirmed.
Patrick has said he favors appointing a "caretaker" who won't run in the special election. Markey won't ask for the appointment, the person familiar with Markey's decision told the Globe.
Brown, who won his seat in a 2010 special election after Kennedy, a Democratic titan, died in office, hasn't said if he will run again.
Markey passed up an opportunity to succeed Kennedy in that special election.
Markey first won his House seat in a 1976 special election after Rep. Torbert Macdonald, D-Mass., died in office. Markey has been re-elected 18 times with no substantive opposition. He has sometimes run unopposed and has never received less than 62 percent of the vote in a re-election bid.
He won last month with 76 percent of the vote. He also has $3.1 million in his campaign account, the Center for Responsive Politics says. That money can all be transferred to a Senate campaign.