President Trump drew criticism Wednesday from the incoming chairman of the House Armed Services Committee for announcing plans to replace outgoing Defense Secretary James Mattis earlier than previously expected.

Rep. Adam Smith, Washington Democrat, said Mr. Trump has placed the United States “in a riskier position” by appointing Deputy Defense Secretary Patrick Shanahan to lead the Pentagon starting Jan. 1, ousting Mr. Mattis from that role nearly two months ahead of schedule.

“It is a mistake for President Trump to deny the country an opportunity to have a stable transition to a new Secretary of Defense in this fashion,” Mr. Smith said in a statement. “I believe Deputy Secretary Shanahan will be able to successfully serve as acting secretary of defense. His expertise in management, research and development, acquisition and procurement are valuable and conducive to the operations of the Pentagon.

“However, he does not have the comprehensive understanding of global national security threats that Secretary Mattis does,” the congressman continued. “Throwing him into the role of acting secretary with no notice in this way unnecessarily places the United States in a riskier position.”

The White House did not immediately return a message seeking comment.

Mr. Mattis, 68, a retired four-star general and Mr. Trump’s first secretary of defense, announced his resignation Thursday in a letter citing irreconcilable differences between himself and the president, making him among the latest additions to a growing list of Cabinet officials to leave the administration less than two years since Mr. Trump took the oath of office.

“Because you have the right to have a secretary of defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” Mr. Mattis wrote to Mr. Trump.

Mr. Trump initially said that Mr. Mattis would retire at the end of February prior to announcing over the weekend that Mr. Shanahan, a former Boeing executive, will begin work at the start of 2019.

“The president has given no reason why Mattis can’t stay until the end of February as he planned,” Mr. Smith responded Wednesday. “We face too many challenges and too complex a threat environment to add to our risk by leaving the country without an experienced national security leader in the seat.”

Mr. Smith, 53, is slated to replace Rep. Mac Thornberry, Texas Republican, as chairman of the House Armed Services Committee when Democrats gain control of the chamber next month. Mr. Thornberry’s office directed requests for comment to committee representatives, who did not immediately return messages Wednesday.

Mr. Mattis announced his resignation last week shortly after Mr. Trump unveiled plans to withdraw U.S. troops from Syria, contrary to the recommendations of several advisers including his defense secretary. Brett McGurk, the special presidential envoy for the global coalition to counter the Islamic State terrorist organization, subsequently resigned a day after Mr. Mattis, and multiple news outlets reported that both of their departures were spurred by Mr. Trump’s policies involving Syria.

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