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.