B2K singer Raz B and his wife, Mona Li, announced the devastating loss of their baby son Astro Monte Thornton … just three months after he was born.

Sources close to the family tell TMZ … Astro died earlier this week, and his death was unexpected and sudden.

The cause of the infant’s death is unclear.

A family spokesperson tells us … “It is with heavy hearts that we announce the unexpected passing of Astro Monte Thornton, beloved son of Mona and De’Mario ‘Raz B’ Thornton of the R&B supergroup B2K. The family asks for privacy as they navigate this difficult time. They truly appreciate your love and support.”

Raz announced the soon-to-be arrival of Astro on May 18, writing on Instagram … “Right now we’re at the hospital getting ready to welcome our newest addition, Astro … another baby boy to the Thornton family.”

The couple — who also share son Versai CarterThornton — also shared a sweet post from Versai’s Instagram account, which they manage, on May 28, writing … “Best week ever … We welcomed my new baby brother Astro into the family, and it already feels amazing being a big brother.”