Surfboards are sandwich-like composite structures composed of polymeric cores and composite skins. Their performance depends on both hydrodynamic features and mechanical response, particularly flex. While hydrodynamic aspects, strongly influenced by board shape and fin design, have been widely studied, the mechanical behavior of surfboards and fins remains difficult to quantify and systematically tailor, with relatively few experimental or mechanics-based studies available. Surfboard fins play a key role in maneuverability and control by shaping the hydrodynamic forces acting on the board. Existing fin-design literature is largely dominated by lift and drag optimization, often relying on computational fluid dynamics workflows, with limited standardization of structural benchmarks and little attention to the mechanical response of the fin itself. Recent studies have begun to connect hydrodynamics, manufacturing, and on-field behavior, including additively manufactured fins, in-situ flex measurements, and parametric links between fin design and surfing performance. Despite these advances, a mechanics-driven approach to tailoring fin flex and dynamic response within realistic manufacturing constraints remains underexplored. In this work, a surfboard fin concept is introduced in which the conventional bulk core is replaced by an additively manufactured architected lattice, enabling independent tuning of stiffness, mass distribution, and dynamic behavior while preserving manufacturability. A multiscale numerical framework is developed to link lattice unit-cell geometry to overall fin performance. Periodic finite element models are used to extract effective constitutive properties as functions of lattice topology, relative density, and grading strategy. These properties are then mapped onto a full fin model to evaluate natural frequencies and mode shapes, as well as stiffness and strength indicators under representative combined bending–torsion loading. The resulting design maps enable direct, mass- and envelope-constrained comparisons between lattice-core and conventional fins, providing a mechanics-based route to tunable dynamic behavior in sports engineering applications.