Global warming-induced temperature increases adversely affect plant growth, exacerbating agricultural risks and threatening global food security. These challenges are particularly acute for important crops like non-heading Chinese cabbage (NHCC; Brassica campestris L. syn. B. rapa), a nutritionally and economically significant leafy vegetable cultivated worldwide. While breeding for thermotolerance has been explored to stabilize summer supplies, these efforts remain insufficient under complex field conditions, necessitating cost-effective and practical alternatives. Here, we demonstrate that biochar derived from enoki mushroom (Flammulina velutipes) spent mushroom substrate (SMS-BC) significantly alleviates heat-induced growth suppression in greenhouse-cultivated NHCC. SMS-BC alters the soil nutrient profile and enhances available phosphorus and potassium through its unique feedstock properties, including its filamentous architecture and surface functional groups. Furthermore, SMS-BC may reshape microbial community composition of NHCC under heat stress. Our findings establish SMS-derived biochar as an effective soil amendment for alleviating heat stress-induced growth suppression in NHCC.