Doubly stranded pairs of isotactic (it-) chains of poly(methyl methacrylate) (PMMA) coiled inside of the helical cavity of syndiotactic (st-) PMMA chains generate a crystalline triple-helix stereocomplex, but its high brittleness limits its application. Here, we present a strategy to toughen up such stereocomplex via construction of thermoplastic elastomer (TPE)-based all-methacrylic stereoregular triblock co-polymers or toughen up such TPEs via stereocomplexation. Specifically, the stereospecific and living coordination-addition polymerization mediated by a chiral zirconocenium catalyst was utilized for the successful synthesis of isotactic ABA triblock co-polymers (it-MRM, M-n up to 200 kDa, isotacticity [mm] > 95%) with it-PMMA as two outer complexing hard blocks and it-PRMA bearing C4-10 alkyl (R) groups as the center, noncomplexing, low-T-g soft block. Physical blending of it-MRMs with st-PMMA in a 1:1 or 2:1 st/it-PMMA ratio, when crystallized from tetrahydrofuran solutions, led to stereocomplexed TPEs self-assembled into phase-segregated soft, elastic (T-g from -10 to -40 degrees C) and high melting, stereocomplexed (T-m = 192-199 degrees C) domains, the latter of which serves as reprocessable physical crosslinks. Characterizations of mechanical properties of the resulting stereocomplexed triblock copolymer materials showed a significant toughening of the polymer network compared to the noncomplexed triblock co-polymer, attributed to the ability of dissipating tensile forces (necking) by the st/it-PMMA domains (tensile toughness up to 29 MJ M-3) and strain hardening by the entangled soft it-PRMA domains (elongations from 260 to 800%).