This study investigates the effects of resistant starch (RS) supplementation on weight loss and metabolic health in individuals with overweight or obesity. A randomized, placebo-controlled, crossover trial involving 37 participants found that an 8-week RS intervention significantly reduced body weight, fat mass, and waist circumference compared to a control starch intervention. The benefits were associated with changes in gut microbiota composition, particularly an increase in *Bifidobacterium adolescentis*, which was strongly correlated with improved BMI, visceral fat area (VFA), and subcutaneous fat area (SFA). Mechanistically, RS-induced changes in the gut microbiota altered bile acid profiles, reduced inflammation by restoring the intestinal barrier, and inhibited lipid absorption. The study also demonstrated that RS facilitated weight loss through *B. adolescentis* and that gut microbiota is essential for the action of RS. Additionally, RS-altered gut microbiota alleviated obesity in mice, suggesting that RS-induced changes in the gut microbiota can drive beneficial host outcomes. The findings highlight the potential of RS as a dietary supplement for weight loss and metabolic improvement, with *B. adolescentis* playing a crucial role in these effects.This study investigates the effects of resistant starch (RS) supplementation on weight loss and metabolic health in individuals with overweight or obesity. A randomized, placebo-controlled, crossover trial involving 37 participants found that an 8-week RS intervention significantly reduced body weight, fat mass, and waist circumference compared to a control starch intervention. The benefits were associated with changes in gut microbiota composition, particularly an increase in *Bifidobacterium adolescentis*, which was strongly correlated with improved BMI, visceral fat area (VFA), and subcutaneous fat area (SFA). Mechanistically, RS-induced changes in the gut microbiota altered bile acid profiles, reduced inflammation by restoring the intestinal barrier, and inhibited lipid absorption. The study also demonstrated that RS facilitated weight loss through *B. adolescentis* and that gut microbiota is essential for the action of RS. Additionally, RS-altered gut microbiota alleviated obesity in mice, suggesting that RS-induced changes in the gut microbiota can drive beneficial host outcomes. The findings highlight the potential of RS as a dietary supplement for weight loss and metabolic improvement, with *B. adolescentis* playing a crucial role in these effects.