Previous studies have shown that dietary selenium (Se) deficiency in mice reshapes gut microbiota, exacerbates healthspan deterioration (e.g., type 2 diabetes), and paradoxically activates beneficial longevity pathways. This study demonstrated that dietary Se deficiency accelerated many age-related gut microbial changes in aged telomere-humanized C57BL/6J diabetic mice in a sexually dimorphic manner, with Akkermansia muciniphila showing the greatest enrichment in males. However, dietary Se deficiency did not enrich A. muciniphila in mature or middle-aged male C57BL/6J wild-type mice. Oral gavage of A. muciniphila alleviated Se deficiency-induced type 2 diabetes-like symptoms, reversed mucosal barrier dysfunction and gut inflammation, and resulted in a trend of symbiotic and competitive suppression changes in certain gut bacteria in mature wild-type mice under conventional conditions. The beneficial effects of A. muciniphila appeared to be independent of selenoproteins sensitive to dietary Se deficiency, such as GPX1, SELENOH, and SELENOW, in the liver and muscle. Altogether, these results show that dietary Se deficiency accelerates age-related A. muciniphila enrichment specifically in aged male mice with severe insulin resistance and pancreatic senescence, indicating a potential hormetic response to Se deficiency through reshaped gut microbiota, which alleviates hyperglycemia and partially compensates for healthspan decline.