It is necessary to develop construction materials with a limited impact on the environment. To achieve this objective, juncus maritimus fibres were used as reinforcement in the plaster matrix at percentages of 10%, 20%, 30% and 40% by volume. The aim of this study was to investigate the impact of the addition of juncus maritimus fibres on the behaviour of plaster. The juncus fibres were characterised from a physical, thermal and mineralogical point of view, and the plaster was studied both physically and thermally. The effect of the percentages of juncus maritimus fibres on the fluidity of the plaster was evaluated using the Marsh cone method, , i.e. by measuring the time taken for a known volume of gypsum to flow from a cone through a short tube. The thermal and mechanical properties of the composites produced were determined by the hot disc method and by compression and flexural tests. The results showed that the bulk density and thermal conductivity of the composites were reduced by 6.45% and 10.9%, respectively, for a percentage of 40% compared with the reference sample. The addition of 20% juncus maritimus fibres improved the flexural strength by 7.43% compared with the reference sample, but a decrease in compressive strength was observed due to poor adhesion between the fibres and the gypsum matrix. The microstructures of the composites were determined using the scanning electron microscope method.