Sweet chestnut (Castanea sativa Miller) is a high-value food crop with significant social and economic importance in the mountainous rural regions of Portugal. Its production has also become increasingly relevant in agriculture and forest restoration worldwide, contributing to economic development, food security, and environmental sustainability. However, chestnut yield and nut quality are being compromised by brown rot disease, caused by the fungus Gnomoniopsis (Sirococcus) smithogilvyi, which affects the kernel, resulting in brown lesions in the endosperm and embryo and thereby reducing the economic viability of chestnut production. This study aimed to evaluate the geographical distribution and incidence of chestnut brown rot in orchards located in northeastern Portugal. A total of 51 orchards were selected across different geographical areas of the Terras de Trás-os-Montes Intermunicipal Community (CIM-TTM), including important chestnut-producing municipalities such as Bragança, Parada, Vimioso, and Vinhais. In each orchard, six trees were sampled, and 15 chestnuts were collected from each tree during the 2025 harvest season. The nuts were then subjected to both external and internal visual assessments. The external assessment included the quantification of chestnuts showing signs of pest infestation, damage, or visible fungal growth. The internal assessment involved recording the number of chestnuts exhibiting rot symptoms, as well as the type and severity of rot observed. The results demonstrated the presence of chestnut brown rot in all sampled areas, although incidence and severity varied among locations. Mean disease incidence ranged from 10% to 22%, with the highest infection levels recorded in chestnut orchards located in the municipality of Vimioso. Regarding disease severity, most infected chestnuts showed moderate symptoms, including internal discoloration and partial tissue degradation. In some locations, severe symptoms associated with advanced rot stages were also observed. The highest average severity was recorded in Vinhais municipality, although the orchard with the highest severity level was located in Vimioso. This study confirmed the widespread occurrence of chestnut brown rot in northern Portugal. These findings highlight the need to implement phytosanitary monitoring and disease management strategies to minimise economic losses and improve chestnut quality. Additionally, the epidemiology of the disease will be analysed in relation to edaphic, agronomic, and climatic factors considered in this study.