The singer, 32 - who is currently expecting his second child with wife Kelsey, and is already a proud dad to 15-month old Aurelia Rose - said his family were 'devastated' by the news, but are determined fight it "all the way".
The brain tumour is grade four glioblastoma, which is tragically inoperable, and life expectancy is anywhere from three months to 18 months on average, after diagnosis.
Figures suggest that patients have a 10 percent chance of living five years after their diagnosis.
The singer is now undergoing radiotherapy and chemotherapy so he can support his wife, daughter and unborn child for as long as possible, and to "raise awareness of this terrible disease".
Sharing a picture on Instagram on Monday with his daughter and Kelsey, who is 36 weeks pregnant, he wrote: "Hey guys, you know that we've both been quiet on social media for a few weeks and it's time to tell you why.
"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."
Tom first got the diagnosis after weeks of unexplained seizures. And it was then doctors told him it was "the worst case scenario".
The singer has already told his bandmates, Max George, Siva Kaneswaran, Jay McGuiness and Nathan Sykes, and says they have been a great support.
He hasn't asked for a prognosis, as he's worried he'd end up counting down the days rather than fighting.
The singer is now being treated at King's College Hospital in London, and his wife, Kelsey, is hoping to explore holistic and alternative treatments, too, if need be.
Speaking to OK! Magazine, he said: "I need everyone's love and positivity. I am going to fight this. We are already exploring alternative treatments and looking at clinical trials both here and abroad, and our friends have been doing research.
"There are so many stories of people who were given a bad prognosis and are still here five,10, even 15 years later. We're going to fight this all the way."
Kelsey added: "If someone had told me my life was going to be taking Tom to radiotherapy in London every day and I'd be 36 weeks pregnant at the time, I wouldn't have believed them.
"To be honest, I think I've forgotten I'm pregnant. It's all about Tom at the moment. I'm human and this has been the worst possible news.
"I cry, but I don't do it in front of Tom, I need to be strong for him and positivity is the best medicine right now. Aurelia has been a brilliant distraction in that sense."
When asked if he had got his head around not being there for his children's future, Tom defiantly replied: "No, because I'm going to be here, I'm going to fight this."
You can donate to The Brain Tumour Charity and support people in Tom's position here.