In 2015, ASEAN estab­lished a goal of increas­ing renew­able ener­gy share in its ener­gy port­fo­lio from approx­i­mate­ly 13–23% by 2025. Renew­able elec­tric­i­ty, espe­cial­ly inter­mit­tent and vari­able sources, presents chal­lenges for grid oper­a­tors due to the uncer­tain tim­ing and quan­ti­ty of elec­tric­i­ty sup­ply. Grid flex­i­bil­i­ty, the elec­tric grid’s abil­i­ty to respond to chang­ing demands and sup­ply, now stands a key resource in respond­ing to these uncer­tain­ties while max­i­miz­ing the cost-effec­tive role of clean ener­gy. We devel­op and apply a grid flex­i­bil­i­ty assess­ment tool to assess ASEAN’s cur­rent grid flex­i­bil­i­ty using six quan­ti­ta­tive indi­ca­tors: grid reli­a­bil­i­ty, elec­tric­i­ty mar­ket access; load pro­file ramp capac­i­ty; qual­i­ty of fore­cast­ing tools; pro­por­tion of elec­tric­i­ty gen­er­a­tion from nat­ur­al gas; and renew­able ener­gy diver­si­ty. We find that ASEAN nations clus­ter into three groups: bet­ter; mod­er­ate­ly; and the least pre­pared nations. We devel­op an ana­lyt­i­cal ramp rate cal­cu­la­tor to quan­ti­fy expect­ed load ramps for ASEAN in an inte­grat­ed ASEAN Pow­er Grid sce­nario. The lack of fore­cast­ing sys­tems and lim­it­ed elec­tric­i­ty mar­ket access rep­re­sent key weak­ness­es and areas where dra­mat­ic improve­ments can become cost-effec­tive means to increase region­al grid flex­i­bil­i­ty. As ASEAN pur­sues renew­able ener­gy tar­gets, region­al coop­er­a­tion remains essen­tial to address iden­ti­fied chal­lenges. Mem­ber nations need to increase grid flex­i­bil­i­ty capac­i­ty to ade­quate­ly pre­pare for high­er pen­e­tra­tions of renew­able elec­tric­i­ty and low­er over­all sys­tem costs.