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.

The couple -- who also share daughter Versai Carter -- is asking for privacy during their difficult time ... The Shade Room reported.

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 also shared a sweet post from their daughter'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."

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Raz recently celebrated Father's Day with the family, sharing on their joint Instagram ... "Thank you for your love, commitment and generosity for us and those you care for."

In the accompanying video, Raz B could be seen sweetly holding his infant son.

RIP