A number of billboards have appeared in Cardiff, Swansea and Aberdare calling for the monarchy to be abolished.
The giant posters, one of which includes an image of the Prince of Wales, reads "Wales doesn't need a prince".
The organisation, Republic, are behind the campaign and said it hopes the billboard will "trigger a revitalised public debate about the monarchy and how we're governed."
Republic has raised more than £25,000 for the campaign and argue "The monarchy is wrong in principle, wrong in practice and it's bad for British politics."
The group argue on their website a "hereditary public office goes against every democratic principle".
Further stating that the Queen and the royal family cannot be held to account via the ballot box, unlike political parties and others in power.
The group proposes a new elected head of state should be put in place to "represent our hopes and aspirations - and help us keep politicians in check".
Leader of the Welsh Conservatives Andrew RT Davies argued the campaign is "simply not reflective of Welsh public opinion".
“The Royal Family are extremely popular in Wales, and that is because their presence provides guaranteed stability in the UK and they are fantastic ambassadors for this country.
“The campaign behind these billboards is a fringe movement that is simply not reflective of Welsh public opinion and totally divorced from the people's priorities.”
Clarence House has been approached for comment.