Utilizing mangrove (Sonneratia alba) fruit as an alternative feed source presents a viable avenue for enhancing livestock feed efficiency and sustainability. However, the intrinsic high tannin content of S. alba poses a significant barrier to its digestibility and utility as a feed component. The application of fermentation processes facilitated by Aspergillus niger emerges as a strategic intervention to diminish tannin impediments and elevate the feed material's nutritional profile. This investigation delves into the ramifications of varied fermentation periods with A. niger on the digestibility, nutrient quality, and fermentative by-products of S. alba fruit. Adopting a completely randomized block design, the study administers treatments spanning fermentation durations of 7, 10, 13, and 16 days, scrutinizing their influence on a spectrum of digestibility indices, ruminal fluid properties, microbial protein synthesis, and gaseous production. The findings articulate that protracted fermentation markedly augments the digestibility of dry matter, organic matter, and crude protein, alongside elevations in volatile fatty acids and ammonia levels, while sparing rumen pH and overall gas output from significant alterations. Notably, a 16-day fermentation tenure culminates in optimal feed digestibility and nutritional amelioration, underscoring the potency of extended A. niger fermentation in curtailing tannin contents and fostering the adaptability of S. alba fruit as livestock feed, thereby advocating for advanced, sustainable feed formulation methodologies.