<p>Purpose of this study is to develop longitudinal flatfoot pads using 3D printing technology for support and treatment of the arch. By applying various materials and designs to the pads, the most effective condition was confirmed. The design of the 3D models was inspired by commercially available wool (WL) based pads, and fabrication was carried out through fused deposition modeling (FDM) 3D printing. To manufacture products with different hardness levels, five lattice structures (Voronoi, tetrahedral, kagome, rhombic, and icosahedral) as well as a solid (SL) structure were applied. For materials, thermoplastic polyurethane (TPU), lightweight thermoplastic polyurethane (LW-TPU) and lightweight polylactic acid (LW-PLA) were used to produce flatfoot pads with various lattice configurations. The manufactured 3D printed longitudinal flatfoot pads were first analyzed in terms of morphology and compressive properties. Subsequently, two selected structures were evaluated under both static standing and walking conditions using plantar pressure analysis to identify the most suitable manufacturing conditions for flatfoot orthotic applications. The results showed that, in terms of morphological and compressive properties, ICO-H lattice exhibited superior strength, whereas VOR provided higher flexibility with reduced strength. These mechanical properties contributed to plantar pressure responses during both static and walking motions. In plantar pressure analysis, LW-PLA flatfoot pads exhibited improved pressure distribution and arch support by increasing contact area. In conclusion, 3D printed longitudinal flatfoot pads using LW-PLA pads improved contact area and redistributed midfoot pressure, while 3D printed longitudinal flatfoot pads using LW-TPU provided superior outcomes by reducing localized loading and enhancing arch support. Notably, Icosahedral lattices with LW-PLA designs were most effective for arch stabilization.</p>