SEOUL, South Korea — A transgender woman who was expelled by the South Korean military last year after her gender-reassignment surgery has been found dead at her home south of Seoul, the police said on Thursday.

The authorities said they were investigating the cause of death of the woman, Byeon Hee-su, 23, whose body was found on Wednesday at her home in the city of Cheongju by emergency responders. They were alerted after a local mental health center that had been counseling her reported that it could not contact her.

Ms. Byeon, who had been a staff sergeant in an army tank unit, was discharged from the military in January 2020 following her surgery. She had wanted to continue her service in the army, but a military panel declared her unfit to serve. She became the first active-duty soldier in South Korea to be referred to such a panel because she had had a sex-change operation.

Since her dismissal, Ms. Byeon had campaigned vigorously to be reinstated, arguing that there was no reason she could not fulfill her duties.

“I want to show that I can be an excellent soldier who helps defend this country regardless of my sexual identity,” Ms. Byeon said tearfully at a news conference following her discharge. “Please give me that chance.”

Ms. Byeon’s case laid bare the plight that lesbians, gay men, bisexuals and transgender people often face in South Korea’s socially conservative society, especially in its armed forces. Gay and other soldiers have long complained of discrimination and abuse. Gay men and lesbians are not barred from service, but they have been subjected to investigations by military officials. Transgender people, however, are barred from joining the armed forces, since they are categorized by the military as having mental and physical “disorders.”

In a ruling last year, a district court formally recognized Ms. Byeon as female. After her initial petition to be reinstated was rejected by the military, Ms. Byeon sued the military, arguing that her discharge had been unlawful. The first hearing in the case had been scheduled to take place in a military court next month.

The military expressed its condolences over her death but declined to make any further comments.

Ms. Byeon’s death brought an outpouring on social media from transgender people, who thanked her for speaking out for transgender rights in the face of the social stigma.

“I am truly sorry that we have failed to protect the life you have so desperately wanted,” Jang Hye-young, a lawmaker affiliated with the minority Justice Party, said in a post on Twitter.

Efforts to adopt a comprehensive anti-discrimination law to promote the rights of women and sexual and other minorities have been stymied for years in Parliament as powerful conservative Christian churches have lobbied against it, calling the conduct of L.G.B.T. people sinful.

Ms. Byeon​ joined the military in 2017. She underwent her operation in Thailand while on leave. She ran into trouble afterward, when a South Korean military-run hospital, where she had checked in for post-surgery treatment, said that she was disabled and could be discharged from the army because of the loss of male genitalia from the surgery.

South Korea, which is technically at war with North Korea across one of the world’s most heavily fortified borders, requires all able-bodied men to serve in its armed forces for about 20 months. Women are exempt from conscription but may choose to enlist.

Before her death, Ms. Byeon found significant international backing for her cause.

United Nations human rights officials said in a letter to the South Korean government last July that her dismissal “would violate the right to work and the prohibition of discrimination based on gender identity under international human rights law​.”

​The South Korean government defended the military’s decision, saying that in order to allow transgender people to serve in the military, the country would have to consider how it would affect troops’ combat readiness against North Korea. It also said the nation had to weigh the “effects on personnel morale​”

In December, ​South Korea’s National Human Rights Commission​ called the army’s decision unfair and recommended that it reinstate Ms. Byeon.

Lim Tae-hoon, director of the Military Human Rights Center of Korea, which provided assistance for Ms. Byeon, said after her death, “We pray that Staff Sergeant Byeon Hee-su, a tank driver, will live with like-minded people in the next world where there is no discrimination and hatred.”