One of the fundamental challenges in laminated composites is weak interlaminar adhesion and their susceptibility to delamination, a damage mode that can lead to a decrease in service life across various applications and even sudden catastrophic failure. In this context, the present study evaluates a novel and developing approach to improve the mode I critical strain energy release rate in glass/epoxy laminated composites, namely the use of three dimensionally printed (3D-printed) polymeric interlayers based on water soluble polyvinyl alcohol filament. Notably, the interlayers employed in this study possess a rhombic cellular structure designed to balance the volumetric fraction of resin and reinforcing filament at the delamination interfaces. Experimental results showed that incorporating the 3D printed interlayer increased the maximum load capacity by 22%. In addition, the initiation and propagation values of interlaminar fracture toughness increased by 45% and 87%, respectively, compared with specimens without an interlayer, indicating a substantial improvement in resistance to interlaminar crack growth and the activation of additional energy dissipation mechanisms. Furthermore, the fracture process zone length in specimens containing the interlayer, recognised as an indicator of the extent of crack growth resistance mechanisms, increased by 143%. Fractographic analysis also revealed that, in the presence of the interlayer, the dominant failure mechanism shifted from interfacial failure to cohesive failure, involving interlayer tearing, stretching, and bridging, together with failure within the interlaminar resin. These observations confirm the effective role of the proposed interlayer in suppressing unstable crack propagation while simultaneously enhancing interfacial adhesion and delamination resistance.