He has faced criticism for not telling senior officials about it for days.
He has since apologised for not “ensuring the public was appropriately informed”.
The lag in notifying the White House raised potential national security concerns and issues of transparency within the Biden administration.
The defence secretary sits just below the president in the chain of command for the US military, and is one of the most important members of the president’s Cabinet.
The Pentagon confirmed Mr Austin remained hospitalised on Tuesday. A spokesman did not provide an update for when he would be discharged.
“Secretary Austin continues to recover well and remains in good spirits he’s in contact with his senior staff and has full access to required secure communications capabilities and continues to monitor DoD his day to day operations worldwide,” the spokesperson told reporters.
Doctors for Mr Austin at Walter Reed National Military Medical Center, the nation’s top military hospital, said a check up in December 2023 “identified prostate cancer which required treatment”.
Mr Austin underwent a “minimally invasive surgical procedure” in hospital on 22 December to remove the cancer. He was under general anaesthesia for this procedure.
He returned to Walter Reed on 1 January 2024 after experiencing “complications”, and an evaluation revealed a urinary tract infection. He was transferred to the intensive care unit the next day for further treatment, including for “abdominal fluid collections impairing the function of his small intestines”.
He “never lost consciousness and never underwent general anaesthesia”. Mr Austin’s infection has cleared and “he continues to make progress and we anticipate a full recovery although this can be a slow process”.
Doctors emphasised that cancer was caught early and said that his “prognosis is excellent”.
National Security Council spokesman John Kirby said on Tuesday that President Biden was informed that day about the diagnosis of prostate cancer.
“Nobody at the White House knew that Secretary Austin had prostate cancer until this morning,” he said.
While he emphasised the president’s initial reaction was concern for the secretary’s health, Mr Kirby acknowledged the communications were “not optimal.”
“This is not the way it is supposed to go,” Mr Kirby said.
Mr Biden and Sec Austin have not spoken since their last interaction over the weekend, according to Mr Kirby.
Mr Austin’s deputy, Kathleen Hicks, was not informed of his hospital stay despite being asked to assume some of his responsibilities.
At a Tuesday briefing, Pentagon Press Secretary Air Force Maj Gen Pat Ryder did not provide an explanation for why Mr Austin did not disclose his condition sooner.
“I don’t have that specifically,” he said, but noted that a prostate cancer diagnosis was “deeply personal”. The Pentagon had said this failure occurred because a key staff member had the flu.
Mr Ryder said notification procedures about the hospital stay were under review to “make sure we’re doing better next time”.
On Tuesday, White House chief of staff Jeff Zients directed members of the president’s Cabinet to provide notice when they cannot perform their duties.
