In this study, we investigate the potential use of persimmon peels (PP) in mushroom culture medium for the production of functional mushrooms. Pleurotus eryngii was cultivated in medium supplemented with PP (SMPP) at the following concentrations: 0% SMPP (control), 5% SMPP, 10% SMPP, 15% SMPP, 20% SMPP, or 30% SMPP. The total polyphenol content, DPPH radical scavenging ability, ABTS cation scavenging ability, and reducing power of P. eryngii cultivated in SMPP were investigated. P. eryngii cultured in 20% SMPP produced the highest values for all four measurements. Total polyphenol content, DPPH radical scavenging activity, ABTS cation scavenging ability, and reducing power all increased upon the addition of PP. Based on our results, we can conclude that persimmon peels are a highly valuable supplement for functional mushroom culture medium.