The American musician, 45, is best known as the co-lead vocalist and founding member of the country music trio Lady A, which was formed in 2006.
Posting a video on Instagram filmed alongside his wife Cassie, Kelley captioned the post "a little health update" before announcing he had been diagnosed with multiple myeloma, a rare type of blood cancer.
"I had (a) couple random rib fractures that happened, which sparked my doctor into having me do several different blood tests. And one of the last blood tests we did, they found a blood cancer in there called multiple myeloma, and it's rare," he said.
"I'm young, I feel healthy, and the treatments I will say have just come so far, even in the past five years. Even life expectancy just far exceeds what it used to be and, I don't know, I feel really hopeful. I really do."
The singer ended his video by sharing he was "hopeful" despite the diagnosis.
"Right now I feel great and we've got a new record coming out soon with Lady A and we've got Christmas shows. This first six months of treatment is not going to be too invasive, I believe - I can still work and I think physically I will feel like myself," he said.
"I will have tough days, but I want to keep working. I need to be happy and doing what I love to do, and soaking up as much time with my family as I can."
Kelley's band mates commented on the post, with lead vocalist Hillary Scott writing: "The strongest duo right there! Love y'all so much! You've got this!"
Dave Haywood shared: "Covering you in prayer brother. Love y'all. We're all here for you!"
Lady A - previously known as Lady Antebellum - are next set to perform a series of Christmas shows across the United States in December as part of their This Winter's Night tour.
The band is best known for the single Need You Now, which spent five weeks at number one in the US when it was released in 2009.
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