The lead guitarist and founding member of New Found Glory, Chad Gilbert, has died ... his band announced.

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The band shared the sad news with their fans on Instagram Sunday ... and said they were "devastated and shocked" by his passing.

They wrote ... "On the morning of September 20th, Chad Everett Gilbert passed away peacefully in his sleep. He left this world surrounded by friends and family and took his final breaths with his wife and mother by his side, just as he wished."

Despite the sad news, the band said Chad died "peacefully."

As we previously reported ... Chad underwent brain surgery in March to remove 3 tumors from his brain -- and he previously underwent treatment for a rare adrenal gland cancer known as metastatic pheochromocytoma, which had spread to his spine and lungs.

In their post ... the band also credited his "guitar playing, songwriting, humor, and relentless drive" for shaping the band over the course of their 3-decade existence.

New Found Glory -- whose song "My Friends Over You" is often hailed as their finest -- formed in 1997 and have released 14 studio albums over the years.

Chad was 45.

RIP