Obesity　is　a　major　health　concern　worldwide　and　is　considered　to　be　associated　with　disruption　of　host-microbial　homeostasis,　especially　microbiota　composition　in　the　gastrointestinal　tract.　Use　of　microbiota-directed　foods　or　nutraceuticals　therefore　represents　a　promising　approach　for　the　control　of　obesity.　Fucoxanthin,　a　marine　carotenoid,　has　been　proven　to　be　one　of　the　most　effective　anti-obesity　natural　products.　However,　its　action　mechanism　is　yet　to　be　unraveled,　especially　with　respect　to　its　role　in　the　modulation　of　gut　microbiota　composition.　In　the　present　study,　profiles　of　microbiota　in　both　the　cecal　and　fecal　samples　from　BALB/c　mice　given　respectively　the　following　treatments　were　examined:　normal　chow　diet　(NCD),　NCD　+　fucoxanthin　(NCDF),　high-fat-diet　(HFD),　and　HFD　+　fucoxanthin　(HFDF).　The　results　showed　that　fucoxanthin　supplementation　for　4　weeks　significantly　changed　the　composition　of　both　cecal　and　fecal　microbiota.　In　addition,　a　differential　effect　was　observed　between　the　supplementation　to　NCD　and　to　HFD.　The　changes　in　the　Firmicutes/Bacteroidetes　ratio　and　the　abundance　of　S24-7　and　Akkermansia　were　identified　to　be　among　the　major　gut　microbiota　modulating　events　associated　with　the　anti-obesity　bioactivity　of　fucoxanthin.　Hence,　our　results　suggested　that　fucoxanthin　could　be　a　promising　microbiota-targeted　functional-food　ingredient.