Astro's label Fantagio shared the tragic news with fans Wednesday ... saying the singer "left us and became a star in the sky." The company says it's "deeply mourning" the shocking death alongside his family, droves of fans, and fellow artists.

Fantagio says the family will hold a private funeral with close family, friends, and colleagues.

An official cause of death has yet to be revealed, but South Korean police reportedly found him dead in his apartment in Gangnam-gu, Seoul after allegedly not showing for a recent rehearsal.

Fans of Moonbin know he started working as a model in 2004 when he was just a child ... then transitioned into acting, first grabbing a role in 2009 on the Korean drama "Boys over Flowers."

He then debuted with Astro in 2016 with 5 others -- MJ, Jinjin, Rocky, Cha Eun-woo, and Yoon San-ha. They went on to come out with some hits, including "Candy Sugar Pop" and "Baby." Rocky left the K-pop group in late February of this year.

Moonbin took a brief pause from the band in 2019 due to health-related reasons but made his return the following year. The group had a concert scheduled for mid-May, but that's reportedly been canceled.

He was only 25.