Due　to　low　bioavailability　of　dietary　phenolic　compounds　in　small　intestine,　their　metabolism　by　gut　microbiota　is　gaining　increasing　attention.　The　microbial　metabolism　of　theasinensin　A　(TSA),　a　bioactive　catechin　dimer　found　in　black　tea,　has　not　been　studied　yet.　Here,　TSA　was　extracted　and　purified　for　in　vitro　fermentation　by　human　fecal　microbiota,　and　epigallocatechin　gallate　(EGCG)　and　procyanidin　B2　(PCB2)　were　used　for　comparison.　Despite　the　similarity　in　their　flavan-3-ol　skeletons,　metabolic　fate　of　TSA　was　distinctively　different.　After　degalloylation,　its　core　biphenyl-2,2′,3,3′,4,4′-hexaol　structure　remained　intact　during　fermentation.　Conversely,　EGCG　and　PCB2　were　promptly　degraded　into　a　series　of　hydroxylated　phenylcarboxylic　acids.　Computational　analyses　comparing　TSA　and　PCB2　revealed　that　TSA＇s　stronger　interflavanic　bond　and　more　compact　stereo-configuration　might　underlie　its　lower　fermentability.　These　insights　in　the　recalcitrance　of　theasinensins　to　degradation　by　human　gut　microbiota　are　of　key　importance　for　a　comprehensive　understanding　of　its　health　benefits.　©　2021　The　Authors.　Published　by　American　Chemical　Society.