Solute　segregation　(adsorption)　at　grain　boundaries　(GBs),　which　is　ubiquitous　in　polycrystalline　materials　and　can　remarkably　alter　various　properties,　is　one　of　the　classic　materials　science　problems.　Despite　decades　of　research,　the　understanding　of　the　atomic　level　GB　segregation　structures　is　still　limited,　mostly　to　symmetric　GBs.　Here,　we　combine　aberration-corrected　electron　microscopy　and　first-principles　calculations　to　reveal　a　highly　asymmetric　interfacial　superstructure　in　WC.　Several　striking　features　are　observed　concurrently　at　this　GB.　First,　the　segregations　of　Ti　and　Co　are　highly　asymmetric.　Second,　the　maxima　of　the　Ti　and　Co　adsorption　profiles　are　both　off　the　center　in　the　opposite　sides,　separated　by　an　intermediate　W-rich　atomic　layer　with　much　less　adsorbates.　Third,　accompanying　asymmetric　interfacial　structural　transitions　occur　to　form　a　cubic-TiC-like　interfacial　layer　on　the　one　side　and　a　partially　disordered　Co-rich　segregation　layer　on　the　other　side.　Such　a　highly　asymmetric　interfacial　superstructure　knowingly　differs　from　all　prior　experimental　observations　and　is　beyond　the　predictions　of　any　existing　models.　Thus,　this　observation　extends　both　the　classic　GB　segregation　models　and　the　new　complexion　theory.　First-principles　calculations　further　verified　all　observed　phenomena　and　provided　new　insights　based　on　the　analysis　of　differential　charge　transfer　and　bond　ordering.　A　new　descriptor,　sum　of　bond　ordering,　is　introduced　to　predict　the　segregation　trend　in　complicated　interfacial　structures.　©　2019　The　Royal　Society　of　Chemistry.