He was found unresponsive by his manager at his apartment in Seoul on Wednesday night, South Korean media quoted police as saying.
"He appears to have taken his own life but an autopsy is being reviewed to determine the exact cause of death," police said.
Moonbin "unexpectedly left our world and became a star in the sky", a tribute from his label Fantiago said.
Moonbin had been an actor and model before joining the boy band Astro in 2016, a well-established K-pop group. He also performed with a sub-group called Moonbin & Sanha.
