<p>Groundwater is a crucial resource for human consumption and economic activities, yet it faces significant threats from human activities and climate change. This study investigates the hydrogeological characteristics of the Penha Mountain region in Guimarães, northern Portugal, which is dominated by a fractured granitic aquifer system that plays a vital role in local public water supply. The primary aim is to address knowledge gaps regarding groundwater quality and recharge mechanisms. A total of forty groundwater samples was selected and collected during the wet (November and December of 2021) and dry (June of 2022) seasons of a hydrological year, with a focus on physicochemical parameters. Additionally, sixteen groundwater samples were analyzed for principal chemical elements and environmental isotopes (δ¹8O and δ²H) during the dry season. The findings reveal that groundwater is slightly to weakly mineralized, characterized by low total dissolved solids (TDS: 14.1–59.0 ppm) and pH values ranging from acidic to neutral (pH: 5.0–7.3). Hydrochemical analysis indicates a dominant sodium-chloride water composition, while isotopic data suggest a meteoric origin, primarily linked to local precipitation and a shallow flow regime at altitudes between 455 and 525 m. Overall, this research demonstrates that groundwater in the Penha Mountain region is suitable for various uses, including human consumption, agriculture, and recreation. Furthermore, this study contributes to the development of a hydrogeological conceptual model for the aquifer system, highlighting the role of regional tectonic structures and geological features in water–rock interaction. The findings offer invaluable insights for the sustainable management of groundwater resources and regional environmental risks assessment.</p>