Coprinus cinereus (Schaeff.) S. Gray s. lato, is a warm tropical, robust and fast growing saprophytic edible and medicinal indigenous Tanzanian mushroom. Grass basal substrate was supplemented with nine different Sargassum species supplements referred as (supplement 1, 2, 3, 4, 5, 6, 7, 8 and 9) at seven different concentration rates % (0.5, 1, 2, 5, 10, 15, 20). The combinations were tested for their influence on performance characteristics of Coprinus cinereus (Schaeff.) S. Gray s. lato mushroom namely; basidiocarps numbers, weights, pileus and stipe lengths, mushroom size, production rate and productivity using solid-state fermentation bioreactors (SSFBs). The substrate and supplements pasteurization, spawn preparation and spawn running were carried out following standard methods. The combinations mixtures were inoculated with 5% spawn wet/wet basis. The inoculated mixtures were arranged in a completely randomized design on shelves in the mushroom growing room and incubated at ambient temperature (25±2°C). Manually spraying water on the walls and placing open containers filled with water in the corners of the room controlled the relative humidity of the mushroom growing room. Data were analyzed using SPSS version 15.0. Results showed that regardless of grass substrate, the interaction of nine brown seaweeds Sargassum species supplements with seven varying concentrations of supplements produced extreme significant effect (p<0.0001) on mushroom performance characteristics variable analyzed. Mean durations from pinning to harvestable young immature fruiting bodies ranged from 1 to 3 days. Among nine supplements comparatively overall best mushroom performance characteristics variables analyzed were recorded from supplement 4 (comprised of Sargassum poligocytum brown seaweed, 8 cm tip-part, which, represented the young parts of Sargassum frond) at 15% concentration rate. It gave highest mean number of fruiting bodies of 85.00, highest average mushroom dry weight of 4.81g, highest mean production rate of 2.23 and highest mean productivity P, of 16.69%. On the other hand, supplement 4 recorded highest mean total fresh mushroom weight of 83.43 g at 10% concentration rate while relatively longest mean pileus (cap) of 20.70 mm was obtained at 0.5% concentration rate. The present findings indicate the potential of brown seaweeds supplement 4, which could be recommended for cultivation Coprinus cinereus (Schaeff.) S. Gray s. lato mushroom on grass substrate for culinary, nutriceutical and therapeutic purposes.