Peat moss is widely used as a casing material for Agaricus bisporus. However, its use in Korea depends largely on imports, and environmental concerns associated with peat extraction have increased the need for alternative casing materials. This study evaluated the potential use of spent coir substrate generated after hydroponic tomato cultivation as an alternative casing material. Spent coir substrate was mixed with peat moss at 10–90% (w/w) or used alone, and two A. bisporus strains, ‘Saehan’ and ‘A15’, were cultivated. The peat–coir mixtures (S10–S90) had pH values of 8.1–8.5 and electrical conductivity (EC) values of 0.2–0.6 mS/cm, whereas S100 had a lower pH of 6.9. No consistent increase or decrease in yield was observed with increasing proportions of spent coir substrate. S100 showed the highest cumulative yield over three flushes in both strains, reaching 4.99 kg·m-2 in ‘Saehan’ and 7.92 kg·m-2 in ‘A15’. Days to primordia formation and most fruiting body characteristics showed no significant differences among casing treatments. These results suggest that spent coir substrate from hydroponic tomato cultivation has potential as a peat moss alternative for A. bisporus casing. Further evaluation of physical properties, particularly water retention characteristics and porosity, is needed to determine suitable conditions for its practical use.