This study systematically evaluated the effects of fermented red ginseng by-product (FRGB) supplementation on rumen fermentation characteristics and nutrient digestibility using in vitro experiments. The results demonstrated significant dose- and time-dependent effects of FRGB on gas production kinetics, with the 4% supplementation level (T4) showing the most favourable performance during the mid- and late fermentation stages, reaching the theoretical maximum gas production. Although all treatments showed a characteristic pattern of increasing gas production, the T4 group maintained greater persistence, indicating optimal substrate composition and effective microbial regulation that sustained fermentation activity. Compared with the control, FRGB slightly reduced the initial gas production rate but significantly improved the overall fermentation potential. The T4 group produced the highest propionic acid concentration and the lowest acetate-to-propionic acid ratio, indicating improved energy utilisation efficiency. Nutrient digestibility analyses showed that 3% FRGB (T3) optimised organic matter digestibility and metabolizable energy, while maximising the neutral detergent fibre and acid detergent fibre degradation, indicating enhanced cellulolytic activity. These differential dosage effects showed functional complementarity: 4% FRGB mainly improved fermentation kinetics and energy release, whereas 3% FRGB enhanced nutrient digestibility and fibre degradation. These results collectively suggest that the optimal addition level of FRGB to ruminant diets is 3–4%, depending on specific production objectives. The present study provides data basis for the evelopment of FRGB as a functional feed additive.