Upper Silurian–Lower Devonian ‘Lower Old Red Sandstone’ facies deposits cropping out in southwest Wales are poorly age-constrained and difficult to correlate. Spore assemblages have been recovered from sequences of these deposits belonging to the lower part of the Cosheston Group. The spore assemblages are equated with the breconensis–zavallatus and polygonalis–emsiensis Spore Assemblage Biozones and indicate an early Devonian age (late Gedinnian (late Lochkovian)–Siegenian (Pragian)). The new biostratigraphical data enable correlation of the lower part of the Cosheston Group with the Senni Beds from the main outcrop of the Lower Devonian in South Wales and the Welsh Borderland. In addition, the new age data and stratigraphical correlation place important plant megafossil assemblages from the Cosheston Group and Senni Beds in a more secure stratigraphical framework, thus facilitating comparisons with other Lower Devonian plant megafossil assemblages and enhancing palaeobotanical understanding. Evidence from palynofacies analysis supports sedimentological interpretations which suggest that the ‘Lower Old Red Sandstone’ facies deposits belonging to the Cosheston Group accumulated in a continental fluviatile environment.