3D-printed interlayers, as a novel approach in composites, can improve resistance to delamination; however, the effect of cellular topology on the interlaminar behavior of these materials remains unclear. Accordingly, this study investigates the effect of a circular cellular architecture on the mode I interlaminar fracture toughness (ILFT) of glass/epoxy composites using a 3D-printed interlayer fabricated from polyvinyl alcohol (PVA) filament. Also, to exclusively evaluate the effect of geometry, the performance of this interlayer was compared with that of a diamond-shaped architecture reported in another study, while all other interfering parameters were kept constant to assess only the role of cell topology. The experimental results showed that the circular interlayer improved the initiation and propagation values of ILFT by 127% and 80%, respectively, relative to the control specimen; whereas, compared with the diamond geometry, these improvements were 47% and 6%, respectively. Although the circular interlayer is effective in controlling mode I delamination, its superiority over the diamond geometry is mainly manifested at crack initiation, owing to the elimination of sharp corners and reduction of local stress concentration. During crack propagation, however, the performance difference between the two architectures is limited because the resin-to-filament ratio and interlayer thickness remain constant. Fracture-surface examination further revealed that the specimen without an interlayer exhibited relatively brittle behavior dominated by fiber/resin interfacial separation, whereas the specimen containing the circular interlayer showed a mixed pattern of adhesive failure, cohesive resin fracture, and local damage to the PVA filaments, confirming the activation of crack-deflection and crack-bridging mechanisms.