President Trump threw his support behind Pete Hegseth on Monday and said any concern over his defense secretary’s decision to share military attack plans in a Signal group chat was a “waste of time.”

Speaking to reporters on the White House’s South Lawn after the Easter Egg Roll, Mr. Trump said he had full confidence in Mr. Hegseth.

“He’s doing a great job — ask the Houthis how he’s doing,” the president said, referring to the rebel group in Yemen that the United States targeted in military strikes last month.

The New York Times reported on Sunday that Mr. Hegseth had shared details about those strikes on March 15 — before they happened — in a message thread that included his wife, brother and personal lawyer, according to four people with knowledge of the chat.

The details were essentially the same attack plans that Mr. Hegseth had shared on a separate Signal chat the same day that mistakenly included the editor of The Atlantic.

The decision to close ranks around Mr. Hegseth comes as Mr. Trump has so far been unwilling to fire any of his top officials, no matter what headlines they generate — a marked change from his first term. People who had discussed the matter with the president said he was determined to dig in and wanted to see Mr. Hegseth fight back. Mr. Trump has been adamant, in private, about not giving the “fake news” media the satisfaction of seeing him fire one of his top officials in a scandal.