Dubbed 'Hands off our IRN BRU', the petition has already reached half of its target with 2,669 people having signed it at the time of writing.
It comes in response to the announcement that producer AG Barr are set to halve the amount of sugar in the brand in January, ahead of a government crackdown on the fizzy drinks industry.
The Scots favourite will see its content drop from 10.3g of sugar per 100ml to just 4.7g with low-calorie artificial sweeteners added to take up the shortfall.
The company explained that the move was part of a "long-standing sugar reduction programme" for all of its drinks, which will see the likes of Tizer, Irn-Bru and Rubicon all have less than 5g of sugar per 100ml before the government introduces its levy on sugary drinks in 2018.
A spokesman for the company told the BBC: "Irn-Bru will still be made with sugar. We are not removing all the sugar, but we are reducing the amount."
Posted by a user called Ryan Allen on Change.org, the petition reads: "Barr have announced that from January 2018 they intend to cut 50 per cent of sugar from Irn-Bru in favour of sweetener aspartame.
"I understand that soft drink manufacturers are coming under mounting pressure from government to lower the sugar content of their drinks and that they intend on introducing a 'sugar tax' that would apply to the sales of sugary drinks similar to the way cigarettes and alcohol are taxed.
"I would far rather pay more for a bottle than have an altered recipe version."
It also points that replacement sweetener Aspartame has "often been the subject of studies and intense debate" and is reported to links to "a multitude of ailments".
The petitioner finishes his statement with an impassioned plea: "I believe that a responsible adult should have the choice as to what poisons they want to put in their body and i think that IRN BRU should continue to sell its original full sugar recipe even if that is at a price increase, or risk sales plummeting like they have with lucozade since they cut sugar.
"Don't do it Barrs please have a rethink on this. HANDS OFF OUR BRU!."
A similar petition on 38degrees.org also has 411 signatures of a targeted 500.
https://foodanddrink.scotsman.com/drink/10-things-you-probably-didnt-know-about-irn-bru/
