Sulli, whose real name is Choi Jin-ri, was reportedly discovered Monday in her home south of Seoul by her manager, who went there because she hadn't answered phone calls for hours.

The K-pop star lived alone in the house, and according to one police official ... "It seems that she took her own life but we are also looking into other possibilities." Cops say no suicide note was found and there were no signs of foul play.

Sulli's best known for her time as a member of the girl band f(x), which she joined in 2009. She took a health-related break in July 2014, and left for good in August 2015 to pursue a solo career. She released a single titled "Goblin" in June.

She also acted in several TV dramas and movies, and was a popular feminist voice in the deeply conservative country ... and spoke out against cyberbullying.

Sulli became one of 4 cohosts of a South Korean talk show called "The Night of Hate Comments" in June, in which celebrity guests come on to discuss mean comments posted about them on the Internet.

On the show, Sulli talked about backlash she'd received for her lifestyle and responded to hurtful rumors.

She was 25.