Kurt Jewson's Facebook post, which also shows his colostomy bag, scars and hormone implants, was shared by more than 185,000 people within four days.
The 44-year-old said he had exposed himself in his "tubby, pale & middle aged glory" in the hope "just one other man" might then get symptoms checked.
He also wants to alert GPs.
The married father, from west Cornwall, wrote: "Why am I posting this? Well, in the summer of 2014 I had blood in my urine. Went to the GP and he said that it was probably just an infection and would clear up. It did.
"However, it wasn't an infection. It was a symptom of prostate cancer."
Latest updates on this story and more from Cornwall and Devon
Mr Jewson said if his GP had done a blood test then, his cancer would have been caught early, but he was not diagnosed until going back more than a year later with repeat symptoms.
1 in 8
men will get prostate cancer at some point in their lives.
1 in 4 black men will get the disease at some point in their lives.
70 - 74 is the average age for men to be diagnosed.
10,000 men die from it every year in the UK.
He said he was concerned that more and more younger men are getting the disease, and urged men to visit the Prostate Cancer UK website to learn how to spot the signs.
Heather Blake from the charity said: "Kurt's decision to wise men up by this bold action is hugely inspiring.
"One in eight men will get the disease at some time and by sharing such a candid image in this generous way he's reached tens of thousands with a message that may well save lives."
"More than 200,000 people have already visited our website as a result of his story. Kurt's a total hero," she said.
Mr Jewson said of the impact his post had: "I should have tidied the room and put my number one pants on, I should have made a bit more effort... but I just thought 'let's do it' and if we get one person to check themselves and get diagnosed early... then it would have been worth it".
He added that his GP had admitted it had been a "missed opportunity" and that he did not wish to name him or the surgery.
He has undergone surgery which caused complications, but said his prognosis looked positive.

Barney, a former sailing athlete and coach, worked for the England and Wales Cricket Board before joining GB Hockey as head of talent development in 2013.
He takes over from Danny Kerry, who had been both performance director and interim women's head coach since 2014.
Kerry, who guided Britain's women to Olympic gold in August, said in April he would focus on coaching full-time.
"Our sport is in an amazing place," Barney said.
"After the women's team's success in Rio, we are now presented with a great opportunity."

There was little corporate news about to excite the market, but in the FTSE 250, shares in laundry services group Berendsen jumped 11% after it looked set to be taken over.
France's Elis said it had reached an agreement in principle with the UK firm on key terms of a Â£2.2bn offer.
Shares in Flybe rose 1.5% despite the airline reporting a Â£20m loss.
The regional carrier has suffered after previous expansion plans proved too ambitious, but the company said the current year was going well.
On the currency markets, the pound slipped 0.14% against the dollar to $1.2943, but edged up 0.2% against the euro to 1.1535 euros.
A prostate cancer patient has been hailed a "total hero" after a photo of him in his underwear showing his catheter bag went viral.
[NEXT_CONCEPT]
Ed Barney has been named Great Britain hockey's new performance director - overseeing the men's and women's teams.
[NEXT_CONCEPT]
The London market edged lower, with the FTSE 100 index down 28.64 points to 7,449.98 at the close.