Tom Parker said there has been a “significant reduction” in his brain tumour.
The Wanted star (32) revealed in October that he had been diagnosed with a brain tumour which was “inoperable” and “terminal”.
But he revealed today that he was “responding well to treatment” in a post on Instagram.
Alongside an image of him with his family, he wrote: “SIGNIFICANT REDUCTION! These are the words I received today and I can’t stop saying them over and over again.
“I had an MRI scan on Tuesday and my results today were a significant reduction to the tumour and I am responding well to treatment.
“Everyday I’m keeping on the fight to shrink this bastard!”
The musician thanked the “wonderful” NHS for the care he had received.
He also praised his wife Kelsey Parker, who he labelled his “rock”.
“Friends, family and everyone on this journey with me – you have got me through my darkest days,” he said.
Parker added: “This journey is a roller-coaster that’s for sure.”
The musician previously said he had stage four glioblastoma.
In November, Kelsey told ITV’s Loose Women programme that “staying positive is the only way to get through this”.
She added: “I am a super-positive person and that is what is so hard about this situation because obviously people are looking at me thinking: ‘How is she getting out of bed in the morning?’
“But it’s the only way to actually get through this. I have done a lot of work with children, I have done a lot of stuff about bullying, and I always talk about positive energy.
“I think with this situation, it is the only thing that can actually get me through it each day.”
Boy band The Wanted formed in 2009, with Parker, Max George, Siva Kaneswaran, Nathan Sykes and Jay McGuiness.