Around 60 protesters have set up camp next to the controversial Ffos-y-Fran opencast coal mine, on the edge of Merthyr Tydfil.
Wales's first Climate Camp follows the successful camp and protest at the site of a planned open-cast coal mine in Scotland last week.
The Camp will run until 16th August and protesters say they want to expose the hypocrisy in Welsh government policy on coal mining.
'Ffos-y-Fran is one of the biggest mines in Europe,' said a camp protester.
'The Welsh Assembly Government say they want Wales to be net carbon neutral by 2020 but this newly opened mine flies in the face of that. You can't keep digging up fossil fuels.'
The mine lies less than 50 metres from some homes, despite UK's standard provision of a 500 metre buffer zone, and is bitterly opposed by many local residents.
Campaign groups such as Residents Against Ffos-y-Fran (RAFF) have cited the noise, dust and health effects of the mining.
Protesters say that during its 15-year lifespan, coal from the mine will generate as much CO2 as Mozambique.