The melt compatibility between poly(L-lactide) (PLA) and polyamides (PAs) with related thermomechanical properties is addressed. A particular attention is paid to four commercial PAs with extrusion processing temperatures close to PLA (PA10-10 to PA12). PLA/PA blend morphologies without a compatibilizer are first revealed by scanning electron microscopy. PA12 displays the best droplet dispersion into PLA (Dn 700 nm), whereas a poor interfacial adhesion is attested for PLA/PA10-10 blends. Interfacial tensions corroborate the PLA/PA10-10 incompatibility (γ12 9 mN/m, 240 °C) with decreasing γ12 in the order PLA/PA10-10 > PLA/PA11 > PLA/PA12 (γ12 2 mN/m). Surface tensions confirm the highest compatibility between PLA and PA12. Ductilities, toughnesses, and thermal resistances of PLA/PA blends are evaluated up to 40-wt % PA. Brittle-to-ductile transitions are observed for PA content higher than 30-wt % with the highest ductility for PLA/PA12, in accordance with their enhanced compatibility. Impact strengths display similar trends with a twofold increase for PLA/PA12. An outstanding synergy between PLA and PA is highlighted by dynamic mechanical analyses with heat deflection up to 130 °C for PLA/PA blends. The synergy arises from a peculiar crystallization of PLA in the presence of PA. PLA/PA morphologies/interfaces can be consequently tuned by an appropriate PA choice with interesting improvements of thermomechanical properties for high-performance/durable applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48440. © 2019 Wiley Periodicals, Inc.