WASHINGTON—Congressional leaders have taken the unusual step of increasing the number of female lawmakers who can help judge cases before the House Ethics Committee as Capitol Hill contends with a growing number of sexual harassment allegations.

In recent weeks, Republican and Democratic leaders have designated 10 extra members—five Democrats and five Republicans, including seven women and three men—to be part of the pool of members of Congress who could serve on subcommittees that investigate and punish alleged ethics transgressions.

Typically, 20 members are chosen for the pool at the beginning of a new Congress. It has now been expanded to 30, with a focus on adding more women. The additions came at the request of House Ethics Committee Chairwoman Susan Brooks, according to a congressional aide.

The original pool had five women and 15 men. It now comprises 12 women and 18 men with Republicans this week adding four more women, while Democrats named three more women.

To date, all of the sexual-misconduct allegations in Congress have been lodged by women against male lawmakers.

A spokeswoman for House Speaker Paul Ryan said: “Speaker Ryan believes it is critical to not only have adequate resources to conduct investigations on the Ethics Committee, but also that those involved include more equal representation of women and men.”

The original pool of 20 had five women and 15 men. Republicans this week added four more women to the expanded ethics pool, while Democrats named three more women. The pool now comprises 12 women and 18 men.

Overall, about one in 5 House members is a woman.

The House Ethics Committee, unlike other congressional panels, is evenly split between Republicans and Democrats. It typically conducts its work in secret. If tapped, subcommittee members essentially serve as the jury.

The panel on Friday announced it would investigate sexual harassment allegations against Rep. Ruben Kihuen, a Nevada Democrat. Last week, the panel opened sexual harassment investigations into Republican Reps. Blake Farenthold of Texas and Trent Franks of Arizona.

Mr. Franks said Dec. 7 he would resign; such an announcement usually leads to the closure of such an investigation. Mr. Farenthold announced he would not run for re-election, and typically an investigation would continue while he is in office.

The subcommittee announced Wednesday to investigate Mr. Farenthold was made up of two men and two women.

In the Kihuen case, two women have accused the lawmaker of unwanted sexual advances. Mr. Kihuen denies the allegations and has resisted calls from House Minority Leader Nancy Pelosi (D., Calif.) to resign.

“As I’ve said previously, I intend to fully cooperate, and I welcome an opportunity to clear my name,” Mr. Kihuen said through a spokeswoman.

A Nevada lobbyist told the Nevada Independent that the freshman lawmaker made repeated unwanted sexual advances toward her in 2013, while he was a state senator. The story was published on Wednesday, prompting new calls for Mr. Kihuen to step aside from his colleagues. Earlier this month, his former campaign finance director told BuzzFeed that the freshman congressman propositioned her for dates and sex.

Mr. Kihuen, a freshman in the House, faces a tough battle for re-election next year in a competitive district. After the first allegations surfaced, the House Democratic campaign arm removed Mr. Kihuen’s name from its 2018 “Frontline” program aimed at protecting incumbents.

Mr. Farenthold announced Thursday that he will not run for re-election following revelations U.S. taxpayers had funded a settlement with a woman who accused him of sexual harassment. Mr. Farenthold denied the harassment but has acknowledged tolerating an unprofessional environment in his House office.

Mr. Franks stepped down after allegations surfaced that he had asked two women staff members to bear a child for him and his wife. Mr. Franks apologized.

Write to Natalie Andrews at Natalie.Andrews@wsj.com and Byron Tau at byron.tau@wsj.com