ABSTRACT This study investigates the role of interfacial shear strength (ISS) in governing the mechanical behavior of fiber‐reinforced composites through an experimental approach. The research explores how ISS influences stiffness, strength, and fracture mechanisms and involves an experimental campaign in three composite systems, Basalt/Vinylester (BV), Glass/Elium Arkema (GE), and Glass/Epoxy Liquid (GEL), examined in untreated and atmospheric plasma‐treated conditions. ISS is quantified based on push‐out tests, while the composites' macroscopic mechanical performance is assessed using three‐point bending and interlaminar fracture toughness (Mode‐I and Mode‐II) tests. Plasma treatment modifies interfacial shear strength in a system‐dependent manner, enhancing ISS and flexural performance in the Glass/Elium composite, while reducing ISS and fracture toughness in the Glass/Epoxy and Basalt/Vinylester systems. These findings demonstrate that interfacial optimization must be aligned with matrix ductility and dominant failure mechanisms. This work provides an integrated experimental assessment combining surface modification, direct ISS quantification, and multiscale mechanical testing across distinct composite architectures, highlighting the existence of an optimal interfacial strength window rather than a universally beneficial increase in ISS.