Bwlchyrhiw Baptist Chapel near Llandovery will receive£3,000 from the authority's Welsh Church Fund to help with the work, which will see information boards put up, conservation work take place and the site's history publicised.
The chapel was built in 1717, but on the site of a much earlier building, which had been secretly used as a place of worship in the 1660s. Baptists, who had been banned from practising their faith after a split from the mainstream church, used to sneak to the site to worship. It was then called Graig Yr Eglwys, and was little more than a large cave.
The donation is part of a£17,010 donation from the authority's Welsh Church Fund to churches and organisations across the county. The grants were agreed in an education and community services director / cabinet meeting on Monday.
Other awards included a£2,600 award for St Michael's Church in Cil-y-Cwm, Llandovery, to help install a boiler, a£1,690 award to Abergwili Church Hall to help replace windows and guttering and a£1,201 award to Emmanuel Baptist Church, Llanelli, to help install central heating.
A£3,000 donation was also included to help St John's Council of Carmarthen buy two defibrilators.
Education and Community Services Cabinet Member, Cllr Mary Thomas said: 'It is good to see that, by having financial support from the County Council, so many organisations in Carmarthenshire can reach their goal and achieve much that they would find difficult otherwise.'