President Trump signed an executive order on Thursday that requires federal buildings in Washington to maintain a classical style of Greco-Roman architecture associated with the marble columns and austere hallways of the Supreme Court and U.S. Capitol.

The new guidelines, which say they are “making federal architecture beautiful again,” also discourage federal construction projects nationwide from choosing modernist styles like Brutalism.

“Because of their proven ability to meet these requirements, classical and traditional architecture are preferred modes of architectural design,” the executive order said. It added, “Major emphasis should be placed on the choice of designs that embody architectural excellence.”

Architects had expected the new rules for some months, ever since the White House released a memorandum in January calling for federal buildings to respect “classical architectural heritage.” It was a throwback to an executive order that Mr. Trump passed in the final weeks of his first administration and was later rescinded by President Joseph R. Biden Jr.

“Architecture should be of its moment,” said Liz Waytkus, the executive director of Docomomo US, a nonprofit dedicated to preserving 20th-century architecture. “It seems the current administration wants to look back and not forward.”

The new guidelines will affect several federal projects, according to Justin Shubow, president of the National Civic Art Society, a nonprofit that helped draft the executive order. He said the competitions underway to design courthouses in places like Hartford, Conn., and Chattanooga, Tenn., would need to follow the executive order. In recent years, courthouses with a modern style have opened in cities like San Antonio, Texas, and Harrisburg, Penn.