Sotomayor said she resigned from the Belizean Grove to prevent the issue from becoming a distraction in her confirmation hearings.
In a letter to Democratic Sen. Patrick Leahy of Vermont and Republican Sen. Jeff Sessions of Alabama, the federal appeals court judge said she is convinced that the club does not practice "invidious discrimination" and that her membership in it did not violate judicial ethics.
But she said she didn't want questions about it to "distract anyone from my qualifications and record."
Federal judges are bound by a code that says they shouldn't join any organization that discriminates by race, sex, religion or nationality.
The Belizean Grove bills itself as women's answer to the 130-year-old all-male Bohemian Club in California. The club owns a 2,500-acre camping area in northern California called the Grove. Chief Justice Earl Warren belonged to the Bohemian Club beginning in the 1940s, before he joined the court and long before the federal judiciary adopted a code of conduct.
"The Belizean Grove is a constellation of influential women who are key decision makers in the profit, non-profit and social sectors; who build long term mutually beneficial relationships in order to both take charge of their own destinies and help others to do the same," the group says on its Web site. There are about 115 members, the club says.
Earlier in the week, Sotomayor defended her participation in the group, telling senators that it involves men in some of its events and that she was unaware of any man who had tried to become a member.
Sotomayor's backers noted that the court's only current woman, Ruth Bader Ginsburg, belongs to the membership-only International Women's Forum. So did former Justice Sandra Day O'Connor, who defended her involvement in all-women groups during her Senate confirmation hearings in 1981.
Supreme Court Justice Anthony Kennedy faced similar questions over his longtime membership in the all-male Olympic Club in San Francisco. Kennedy resigned his membership in October 1987, as he was under consideration for the high court.
Sotomayor also told the senators that the search of documents from her time as a director of a Puerto Rican advocacy group is complete.
Republicans had complained that Sotomayor initially omitted from the records she sent the Senate Judiciary Committee a report she signed urging the Puerto Rican Legal Defense and Education Fund (now known as LatinoJustice PRLDEF) to oppose reinstitution of the death penalty in New York in the early 1980s.
The report, which has since been provided, said, "Capital punishment is associated with evident racism in our society." It noted that African-Americans at the time made up 47 percent of death row inmates, but only 11 percent of the U.S. population.