WASHINGTON — President Trump on Monday will tap Justin Muzinich, a top aide to Treasury Secretary Steven Mnuchin, as the Treasury Department’s deputy secretary, a powerful role that includes overseeing a broad portfolio of policy matters like taxes, financial regulation, trade and economic sanctions.

The nomination fills a void in the top ranks of the department, which has operated without a full-time deputy secretary since Mr. Mnuchin was confirmed more than a year ago. Mr. Trump’s previous picks, the longtime Goldman Sachs executive James Donovan, and Brian Brooks, a Fannie Mae executive who was not formally nominated, withdrew their names from contention last year. Mr. Mnuchin later said that he was not going to fill the position.

The ascendance of Mr. Muzinich comes as Mr. Trump reshuffles his economic team, most notably replacing Gary D. Cohn with Larry Kudlow as the director of the National Economic Council. Far from a flamethrower, Mr. Muzinich, who spent most of his career on Wall Street, will add a more moderate voice with traditional conservative economic ideas to the table.

Mr. Muzinich, 40, has been serving as Mr. Mnuchin’s tax counselor and played a central role in crafting and negotiating the terms of the $1.5 trillion tax overhaul that Congress passed late last year. He was previously under consideration to be promoted to under secretary for domestic finance at the Treasury Department, but now is in line to be Mr. Mnuchin’s No. 2.