Adam Smith, the top Democrat on the House Armed Services Committee, said he was shocked by the appointment.
“I confess, I did not know who Pete Hegseth was until about 20 minutes ago,” Mr Smith told reporters.
“He does not seem to have much of a detailed background in (Department of Defense) policy.
“The lack of experience is concerning.”
Mr Smith said the Pentagon was the “biggest bureaucracy in the world” and a “hard thing to run”.
“I think it’s going to be a challenge,” he said.
By contrast, House Speaker Mike Johnson hailed Mr Trump’s selection, telling AP he believes Mr Hegseth “brings a lot to the table”.
“He’s got experience and I think he’ll be reform-minded in the areas that need reform,” he said.
The appointment of Mr Hegseth followed a series of other nominations on Tuesday, as Mr Trump assembles his Cabinet.
Earlier in the day, he announced that John Ratcliffe, a former director of national intelligence, would lead the Central Intelligence Agency.
Mr Ratcliffe rose to attention in 2019 as an ardent defender of Mr Trump during the House’s first impeachment proceedings against the then president. He was a member of Mr Trump’s impeachment advisory team and strenuously questioned witnesses during the impeachment hearings.
“I look forward to John being the first person ever to serve in both of our nation’s highest intelligence positions,” Mr Trump said in a statement.
“He will be a fearless fighter for the constitutional rights of all Americans, while ensuring the highest levels of national security, and peace through strength.”
Completing the series of announcements on Tuesday, Mike Huckabee, the Arkansas governor was named as ambassador to Israel and Steven Witkoff, a long-time friend of Mr Trump, was made a special envoy to the Middle East.
The president-elect also named Bill McGinley, his Cabinet secretary in his first administration, as his White House counsel.
