Gov. Philip D. Murphy of New Jersey has selected George Helmy, his former chief of staff, to serve out the remainder of the term of Senator Robert Menendez, who was convicted of corruption and is stepping down next week, according to three people familiar with the decision.

Mr. Murphy plans to formally announce the selection in Newark on Friday, according to the three people, who spoke on condition of anonymity because they were not authorized to discuss the choice. Mr. Helmy, one of the governor’s longest-serving inner-circle loyalists, is expected to be sworn in when the Senate returns from its summer recess in September. The term ends in January.

Mr. Menendez, a Democrat, was found guilty last month of taking bribes and acting as an agent of a foreign government after a two-month trial in Manhattan. He plans to appeal the verdict, but has said he intends to leave office on Aug. 20 to avoid becoming a distraction to the Senate’s “important work.”

In tapping Mr. Helmy for the temporary position, Mr. Murphy, a Democrat, bypassed several women who were under consideration. He also passed over Representative Andy Kim, a third-term congressman and the Democratic nominee for Senate, who won the primary with 75 percent of the vote.

Mr. Kim, 42, is considered the favorite in the November race against Curtis Bashaw, the Republican nominee. Being appointed to the job several months early could have given Mr. Kim a seniority edge and a possible head start toward preferred committee assignments. But Mr. Murphy’s closest advisers had said from the start that the governor was unlikely to select Mr. Kim, who for months was locked in a bruising competition for Mr. Menendez’s seat with the governor’s wife, Tammy Murphy.

Mr. Helmy has worked for both Senator Cory Booker and former Senator Frank Lautenberg, experience the governor’s aides said was important to Mr. Murphy.