Laminated composite materials are susceptible to damage, such as delamination, due to poor out-of-plane properties and inadequate adhesion at the layer interface. Using the interlayering method is an effective way to enhance resistance against delamination in these materials. However, interlayer synthesis methods have been limited and sometimes very expensive. Therefore, this study investigates the impact of novel 3D-printed polyvinyl alcohol (PVA) interlayers on the mode II interlaminar fracture toughness in glass/epoxy laminated composites. Notably, the interlayers feature a geometrical structure consisting of square cell shapes, allowing the filament to have an equal volume percentage to the resin in the interlayer. To achieve this, end-notch flexure specimens (ENF) were prepared, and the mode II interlaminar fracture toughness was calculated using the compliance-based beam method (CBBM). The specimens' crack growth resistance curve (R-curve) analysis revealed that using the desired 3D-printed interlayer increases the mode II interlaminar fracture toughness by 73%. Furthermore, the behavior of crack growth resistance in laminated composites reinforced with 3D-printed interlayer shows an increasing trend. The mode II fracture surface analysis of reinforced specimens demonstrates the influence of square cell shapes in creating shear hackles, increasing surface friction at the interface of laminated composite layers, crack pinning mechanism, and changing the direction of crack growth, ultimately leading to increased resistance to crack growth.