Pete Alonso Brings Cancer-Stricken Mets Fan To Tears With Surprise Video

WARNING: THIS VIDEO WILL MAKE YOU CRY!!!

NY Mets superstar Pete Alonso surprised a sick fan with a video message -- and the gesture was so heartfelt, it brought her to tears!

The story behind the footage is touching ... shortly after 82-year-old Kathleen Selig tragically learned she was given just weeks to live after a small cell lung cancer diagnosis, her granddaughter reached out to the Mets over social media.

18-year-old Ally Henglein explained of her "Gammy," "She is a diehard Mets fan and has been since the start."

"The Mets have helped her through the lonliest of times," Henglein wrote. "So much so, she has asked us to spread her ashes at Citi Field when the time comes."

So, how did NY respond? Not only did its best player make her a personalized vid ... new manager Luis Rojas also surprised her with a phone call -- and her reaction was priceless.

You can see in the footage ... the woman tears up with joy over seeing Pete's face -- and then is overcome with emotion when Rojas gets on the phone.

It's super touching ... and the Mets clearly couldn't have been happier to do it for their biggest fan.

As for the aftermath of the amazing moment, Selig's granddaughter wrote, "I’ve never seen her so overcome with emotion. I know she’ll fight to see opening day after all of this love and support."