Earlier in the day, by a 90-8 vote, the U.S. Senate confirmed Esper to serve in the office.
Esper officially became the defense secretary when the president signed the appointment paperwork preceding the formal swearing in.
Esper became acting defense secretary last month after then-acting Defense Secretary Patrick M. Shanahan stepped down from consideration. President Donald J. Trump formally nominated Esper for the top Pentagon job July 15.
Once Esper was formally nominated for the position, he stepped back into his former position as Army secretary. By law, he could not continue to serve as acting defense secretary while under consideration for the permanent job. Navy Secretary Richard V. Spencer became acting defense secretary on July 15.
After Esper was confirmed as defense secretary, Trump this morning formally nominated David L. Norquist for the Pentagon's No. 2 job as deputy defense secretary. Norquist will appear before the Senate Armed Services Committee tomorrow for his confirmation hearing.
Norquist has been performing the duties of the deputy defense secretary. One of the very first things Esper planned to do as secretary of defense was delegate all duties of the deputy secretary to Spencer, chief Pentagon spokesperson Jonathan R. Hoffman told reporters this morning.
When that happens, he added, Norquist will cease to perform the duties of the deputy secretary, and he will solely serve as the Defense Department's comptroller and chief financial officer while under consideration by the Senate.
This is done out of deference to the Senate, to not presume confirmation, Hoffman said.
"Mr. Spencer's leadership during this transition period has allowed the department to continue business as usual," he said. "We are fortunate to have such a talented service secretary who's fully capable of assuming these duties during this unprecedented period of transition."