Jenny Lloyd, of Friends of the Earth Cymru, said the council’s decision was “great news”.
“This brings to a close years of campaigning, but it also brings a stop to Wales’ last opencast coal mine,” she said.
“It’s an opportunity now for that land to be remediated for nature and the community.”
Coal Action Network said there were now “no live applications for new coal mines” in the UK and that the decision reflected “a clear, strategic commitment to climate leadership, rare habitat protection, and safeguarding the health of surrounding communities”.
“We’ll continue to engage with the local authority on the restoration to ensure it is delivered to the standard promised,” added Daniel Therkelsen from the campaign group.
Carmarthenshire council said the application had been refused as it was contrary to the policies set out within Carmarthenshire County Council’s Local Development Plan.
Bryn Bach Coal Ltd has been asked to comment.
In its application, the company had argued it had developed “a niche non-thermal market for premium quality anthracite”.
It said there was considerable demand for its coal from industries ranging from water filtration and battery production to green steelmaking.
The company has six months to appeal the decision.