Ronald P. Wertheim, 48, a member of the federal Merit Systems Protection Board, will be nominated by President Reagan to become a judge of the D.C. Superior Court, the White House announced yesterday.

A resident of Northwest Washington, Wertheim is to be nominated for a 15-year term to fill the vacancy created when Judge James A. Belson was named to the D.C. Court of Appeals.

Before joining the Merit Systems Protection Board, Wertheim was in private practice here with the firm of Ginsburg, Feldman and Bress.

He was adviser to the secretary of Defense for Law of the Sea Negotiations and alternate U.S. representative at the United Nations Conference on the Law of the Sea from May to October 1977.

From 1964 to 1966, he was deputy general counsel of the Peace Corps and from 1966 to 1968, he was Peace Corps director in northeast Brazil.

Before joining the Peace Corps, he was an associate professor of law at the University of Virginia. From 1959 to 1961 he was assistant public defender in Philadelphia. He practiced trial law there from 1957 to 1959.

A 1954 graduate of the Wharton School of Finance and Commerce at the University of Pennsylvania, Wertheim graduated in 1957 from the university's law school where he was editor-in-chief of the law review.

In 1962, he received a diploma from the Hague Academy of International Law.

In 1979, Wertheim represented former Sen. Herman E. Talmadge (D-Ga.) during an investigation by the Senate Ethics Committee into allegations of financial misconduct against Talmadge.

Wertheim also represented former assistant attorney general Robert C. Mardian in his successful appeal from a 1975 conviction in the Watergate cover-up conspiracy.