Brad Arnold -- lead singer of 3 Doors Down -- has cancer ... and, he revealed the news in an emotional social media post.

The musician told the band's many fans that he was diagnosed with clear cell renal carcinoma, a form of kidney cancer. Arnold then says the cancer metastasized to his lungs ... a stage 4 diagnosis.

Brad says he's trying to stay positive ... giving all the glory to God -- adding his faith means he's not scared at all.

That said, Arnold tells fans this summer's 3 Doors Down tour can't go on ... and, he apologizes to all the fans who were excited to cheer them on.

BA finishes off the message by saying he's adopting the band's song "It's Not My Time" as his own personal anthem as he fights off the disease.

Arnold cofounded 3 Doors Down in 1996 when he was just a teenager. He wrote arguably the band's biggest song -- "Kryptonite" -- while still in high school.

3 Doors Down's first album "The Better Life" went seven-times platinum ... and their following three albums all attained platinum status as well.