The 32-year-old singer, who is father to 15-month-old daughter Aurelia with wife Kelsey and has another baby on the way, told Britain's OK! magazine that he has stage four glioblastoma - an inoperable tumor.
"I'm still in complete shock, it's so much to take in," he told the publication.
His wife Kelsey added that she and Tom had an "inkling something was wrong" in July when he suffered a seizure and was put on a waiting list for an MRI scan. Six weeks later, during a family holiday, he had a second, more serious seizure and was rushed to the hospital, where he was told the devastating diagnosis.
"You never think this will happen to you," Tom concurred.
While the average life expectancy for a stage four glioblastoma is between three to 18 months from the time of diagnosis, Tom added that he opted against asking how long he's got left.
Tom also opened up about the diagnosis in a post on his Instagram page, telling fans, "There's no easy way to say this but I've sadly been diagnosed with a Brain Tumour and I'm already undergoing treatment."
"We decided, after a lot of thought, that rather than hiding away and trying to keep it a secret, we would do one interview where we could lay out all the details and let everyone know the facts in our own way. We are all absolutely devastated but we are gonna fight this all the way. We don't want your sadness, we just want love and positivity and together we will raise awareness of this terrible disease and look for all available treatment options."
"It's gonna be a tough battle but with everyone's love and support we are going to beat this. Tom and Kelsey xxx."