Pentagon spokesman Pat Ryder in a statement on Sunday said Deputy Secretary of Defense Kathleen Hicks "assumed the functions and duties" just before 5:00 pm.
The United States Secretary of Defense Lloyd Austin has transferred his duties to his deputy after he was admitted to a critical care ward with a bladder issue, as he battles prostate cancer.
Pentagon spokesman Pat Ryder in a statement on Sunday said Deputy Secretary of Defense Kathleen Hicks "assumed the functions and duties" just before 5:00 pm.
He also added that the White House and Congress had been informed, News.com reports.
Austin was "admitted into the critical care unit at Walter Reed National Military Medical Center for supportive care and close monitoring," the Pentagon said in a statement, citing his doctors.
The latest health scare came weeks after it emerged that Austin, 70, had kept previous hospital stays secret and had not immediately informed US President Joe Biden of his cancer diagnosis, sparking criticism as the United States faces crises in the Middle East and Ukraine.
Austin effectively vanished from the public eye for treatment for prostate cancer in December and again in January after suffering complications from the procedure.
This time, the public was alerted around two hours after he was sent to the hospital on Sunday afternoon.
Austin "was transported by his security detail" to the hospital, Ryder said in the earlier statement.
Ryder noted initially that the defense chief brought along classified communications systems and would be retaining "the functions and duties of his office."
However, the announcement that Hicks would instead be taking over came just a few hours later.
"At this time, it is not clear how long Secretary Austin will remain hospitalized," said the latest statement, attributed to Dr John Maddox and Dr Gregory Chesnut of the Walter Reed National Military Medical Center in Maryland.
"The current bladder issue is not expected to change his anticipated full recovery. His cancer prognosis remains excellent."
Austin apologized this month after coming under heavy political fire for keeping the previous hospital stays secret.
"I should have told the president about my cancer diagnosis," he told journalists on February 1.
At the time, he said he was still in recovery, suffering from leg pain and using a golf cart for transportation inside the Pentagon.
The top US defense official is also a key figure in attempts by the Biden administration to maintain support for Ukraine's fight against Russia's invasion, as Republican members of Congress refuse to authorize new funding for military aid to Kyiv.
Various Republican lawmakers previously called for Austin to be sacked but Biden, while lamenting the Pentagon chief's lapse in judgment, has said he remains confident in his defense secretary.
Austin has gained a reputation as a largely apolitical public servant who is most comfortable with US troops.