Sulli, a former member of K-pop girl group f(x), was found dead yesterday at a house in Seongnam, Gyeonggi province.
The singer and actress, whose real name is Choi Jin-ri, was 25.
South Korea's media reported that the police have confirmed Sulli's death. The Korea Herald said her manager found her dead on the second floor of the house.
He had gone there because he could not reach Sulli after their last phone call on Sunday evening.
The police are investigating the case. "It seems that Choi was living alone at the house," a police official said. "It looks like she ended her life but we will investigate, (we are) open to all possibilities."
News of Sulli's death has shocked K-pop fans, with her name trending worldwide on Twitter.
The entertainer first gained prominence as a child actress playing the role of Princess Seonhwa of Silla in the 2005 period series Ballad Of Seodong.
She later debuted as an idol under SM Entertainment as part of girl group f(x) in 2009. She took a hiatus from the group in 2014, citing malicious online comments targeted at her. She left the group officially in 2015, but had remained with SM Entertainment.
As an actress, Sulli was known for series such as To The Beautiful You (2012). She made a cameo appearance in this year's fantasy South Korean drama Hotel Del Luna and was recently cast in the upcoming sequel of anthology series Persona, which was set to be released next year. Persona 2 was to be her first major acting role in two years.
She attracted controversy in 2014 when she admitted to dating hip-hop artist Choiza, 14 years her senior, as romantic relationships are seen as a no-no in the highly controlled world of K-pop.
During their relationship, which ended in 2017, she posted photos of the two of them kissing and going out on dates. Netizens criticised her posts, calling them provocative.
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Sulli was also in the news earlier this month over a wardrobe malfunction. She was getting her hair and make-up done in an Instagram Live broadcast and donned a loose robe which exposed her breasts.
Asked by netizens why she went bra-less, she said: "I don't get what's wrong. This is my personal freedom."
But her take-charge, confident ways apparently masked another side of her character.
Recently, while shooting an episode of JTBC's Reply Night, she said: "I think everyone has different sides to them. The real (me) has a darker side, but the celebrity Sulli always has to act like she's cheerful.
"I asked for a lot of advice from people around me because I wondered if I was lying to people. All people have a darker aspect to them, but they just act like they don't. We're living ambiguously."
Her death comes less than two years after that of another SM Entertainment artist - Shinee's Jonghyun - who committed suicide at age 27 in December 2017.
Jonghyun had been battling depression before his death, which prompted other K-pop artists like BTS' rapper Suga to speak up about mental health awareness.
South Korea is known for having some of the highest suicide rates among developed nations, and its celebrities are not immune.
Actress Jeon Mi-seon died in June in a suspected suicide case while Winter Sonata (2002) star Park Yong-ha killed himself in 2010.
The entertainment industry in South Korea, especially K-pop, is notorious for its pressure-cooker environment, where idols train for years before debuting, are given meagre allowances and have gruelling performance schedules.
K-pop artists, who often have official social media accounts to promote their work and engage with fans, also frequently face harassment and bullying online.
In 2017, a man was arrested for sending death threats to girl group Twice, and an anonymous netizen threatened to shoot BTS' Jimin.
Idols are also routinely criticised for their looks and even the clothes they wear - Mamamoo's Hwasa and solo act Hyuna have been criticised for wearing revealing outfits.