The interlayering method effectively enhances resistance against delamination in laminated composites. However, synthesis methods for interlayers have been limited and, at times, expensive. Consequently, this study investigates the effect of innovative 3D-printed wood–PLA interlayers on the mode II interlaminar fracture toughness (ILFT) of glass/epoxy composites. These interlayers feature a geometric structure comprising rhomboidal cell shapes, enabling the filament to maintain an equal volume percentage to the resin at the delamination interface. To this end, end-notch flexure (ENF) specimens were prepared, and the mode II ILFT was determined using the compliance-based beam method. The experimental results demonstrate a substantial increase in initiation load tolerance (≅32%) due to the 3D-printed interlayer. The R-curve analysis of the specimens with interlayers reveals significant enhancement in critical delamination parameters, including the length of the fracture process zone (≅23%), initiation ILFT (≅80%), and propagation ILFT (≅44%), compared to the samples without interlayers. The fracture surface analysis of the reinforced specimens with interlayers demonstrated that the interlayer positively impacts the delamination resistance of the ENF specimens. They create a larger resin-rich area and increase surface friction at the delamination interface. Also, this facilitates a crack front pinning mechanism and changes the direction of crack growth.