The HuffPost Entertainment team did a collective 'wow' when we laid eyes on these pics of a certain former pop star this morning.
And the gym isn't the only place Jon's been spending quite a bit of time at judging by this snap he uploaded to Instagram.
As well as showing off his impressive new muscles, Jon also got his tats out.
The inkings include a bird and a rose on his newly pumped-up chest and a collection of butterflies on his arm.
Harry Styles would be proud.
Jon's former bandmate, fellow S Clubber Rachel Stevens, recently sparked rumours of an S Club reunion, saying she thought it would be 'fun'.
The 36-year-old - who was recently named as the Sexiest Woman Of All Time in a FHM readers' poll - told Lorraine Kelly that a reunion hadn't been ruled out.
"We keep in touch," she said. "I haven’t seen them for a while but we’re always texting and tweeting and keeping in touch.
"It’s always bubbling in the background talking about us getting back together. Obviously it hasn’t happened yet because I think getting seven diaries together is really difficult.
"Everyone’s off doing their own thing, but it would be fun."
Jon isn't the first star to go from wimp to wow...