Horrible news from NHL superstar David Pastrnak ... the Boston Bruins forward announced Monday his newborn son has tragically died.

The baby, named Viggo Rohl Pastrnak, was just born on June 17 ... but Pastrnak said on his social media page he lost the child just days later on June 23.

"We have an Angel watching over us and we call him SON," Pastrnak wrote. "You will be loved FOREVER."

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