The upper Viséan–lower Serpukhovian Roque Redonde Formation in the southern part of the Montagne Noire, SW France, contains carbonate mounds with predominant micropeloidal facies tens of meters thick representing paleotopographic highs. Calcareous green algae, including representatives of the Dasycladales and Bryopsidales, are relatively common within the mounds (varying in percentages from 1% to 15% of the components, exceptionally reaching up to 40%). Six morphological types of algal remains are recognized within the mounds, and represented by (1) fragmented material, (2) discrete segments, (3) semi-articulated elements, (4) articulated elements, (5) stacked blades, and (6) bushes. Analysis of their preservation state suggests that they mostly constitute autochthonous or parautochthonous assemblages, which were living during the growth of the predominant micropeloidal/clotted fabrics of the carbonate mounds. As such, the carbonate mounds in the Roque Redonde Formation would have developed in an outer platform setting under dysphotic-euphotic conditions. Green algae in the carbonate mounds of the Mississippian confirm that they do not have analogs within the modern deep-water/bathyal carbonate mounds. The abundant green algae in the Montagne Noire is an exceptional case for any geological time period.