The quality of goat milk in extensive grazing systems is closely related to the biochemical composition of the plant species consumed, which varies seasonally. The objective of this study was to analyze the milk quality of Creole goats and the proximate composition of their diet during two seasons (dry and rainy) in a semi-arid ecosystem in Baja California Sur, Mexico. Data was analyzed using a completely randomized design with three replicates. Milk samples were analyzed using MilkoScan (R) Minor and Fossomatic (R) Minor equipment, while plant species composition was assessed using standard laboratory methods. Milk results between seasons showed significant differences (p <= 0.05) in fat (4.72%), protein (3.84%), lactose (4.55%), total solids (15.66%), and non-fat solids (10.45%) content, being higher during the rainy season. The proximate composition of the species consumed by goats also showed significant differences in moisture, ash, lipids, crude fiber, and energy, both between seasons and between species. These dietary changes were shown in the nutritional quality of the milk, which has relevant implications for the production of artisanal dairy products. It is concluded that seasonal variability in diet quality directly influences the physicochemical parameters of goat milk, which are greater during the rainy season. This can be exploited by local producers to improve the added value of their products.