This study investigated whether rosemary extract (RE) supplementation selectively reshapes ruminal butyrate-producing microbiota and enhances functional milk properties in lactating dairy goats. Twenty-four goats were fed a control diet or a diet supplemented with 2.14 g/kg RE (n = 12) during a 42-day feeding trial. Rumen fermentation parameters, bacterial community composition, and milk quality indices - including bioactive proteins, amino acid and fatty acid profiles, and somatic cell count - were analyzed. RE supplementation significantly increased butyrate concentration and tended to elevate valeric acid. The relative abundances of fibrolytic bacteria (Ruminococcus albus and Fibrobacter succinogenes) and butyrate-producing taxa (i.e., Butyrivibrio) were significantly enriched (p < 0.05) in the RE group. Notably, dietary RE markedly elevated milk bioactive proteins, including lactoferrin, α-lactalbumin, and β-lactoglobulin (p < 0.05), and reduced somatic cell count (p < 0.05). These findings reveal a microbiota-mediated pathway through which RE enhances the functional quality of goat milk and support the potential of phytogenic feed additives.