Organic　electrochemical　transistors　(OECTs)　have　attracted　considerable　interests　for　various　applications　ranging　from　biosensors　to　digital　logic　circuits　and　artificial　synapses.　However,　the　majority　of　reported　OECTs　utilize　large　channel　length　up　to　several　or　several　tens　of　micrometers,　which　limits　the　device　performance　and　leads　to　low　transistor　densities.　Here,　we　demonstrate　a　new　design　of　vertical　OECT　architecture　with　a　nanoscale　channel　length　down　to　similar　to　100　nm.　The　devices　exhibit　a　high　on-state　current　of　over　20　mA　under　a　low　bias　voltage　of　0.5　V,　a　fast　transient　response　of　less　than　300　mu　s,　and　an　extraordinary　transconductance　up　to　68.88　mS,　representing　a　record-high　value　for　OECTs.　The　excellent　electrical　performance　is　attributed　to　the　novel　structure　with　a　nanoscale　channel　length　defined　by　the　channel　material　thickness,　which　is　intrinsically　different　from　that　of　conventional　OECTs　with　the　channel　length　limited　by　the　lithography　resolution.　Owing　to　the　low　thermal　budget,　we　fabricate　flexible　devices　on　a　flexible　substrate,　which　exhibit　unprecedented　endurance　characteristics　and　mechanical　robustness　after　1000　blending　cycles.　Furthermore,　the　proposed　device　is　capable　of　mimicking　biological　inhibitory　synapses　for　application　in　intelligent　artificial　neural　networks.　Our　work　not　only　pushes　the　performance　limit　of　OECTs　but　also　opens　up　a　new　design　of　OECTs　for　high-performance　biosensors,　digital　logic,　and　neuromorphic　devices.