Former Texas governor Rick Perry has been confirmed by the Senate as energy secretary under President Donald Trump.
The vote was 62-37 on Thursday.
Mr Perry - who once pledged to eliminate the department - has repeatedly promised be an advocate for the agency and to protect the nation's nuclear stockpile.
He has said he will rely on federal scientists, including those who work on climate change.
Mr Perry also claimed he will work to develop American energy in all forms - from oil, gas and nuclear power to renewable sources such as wind and solar power.
Democrats say they have accepted Mr Perry's disavowal of his 2011 pledge to abolish the Energy Department.
They claim they are worried he may not stand up to GOP proposals to slash the department's budget.