Lightweight design is a key objective in modern mechanical engineering, driven by both environmental and economic considerations. Additive manufacturing (AM) enables material-efficient, load-adapted component designs with a high degree of geometric freedom. In particular, periodic lattice structures can be efficiently manufactured by AM and exhibit excellent stiffness-to-weight ratios, making them highly attractive for lightweight engineering applications. Despite these advantages, additively manufactured lattice structures are still rarely employed in load-bearing components. A major reason is the lack of reliable and standardized methods for their mechanical characterization, especially under cyclic loading conditions. Fatigue behaviour is strongly influenced by specimen geometry, manufacturing-induced defects, and local stress concentrations, which frequently occur at the transition between lattice regions and solid clamping sections and can significantly influence experimental results. This work presents a specimen design concept for cyclic tension–compression testing of lattice structures based on topology optimization, supported by numerical and experimental fatigue investigations. An optimized transition between lattice and bulk material is introduced to reduce stress concentrations and ensure a homogeneous load transfer. The proposed design is evaluated using finite element–based fatigue analyses and experimental results in both the as-built and post-heat-treated conditions, demonstrating improved reproducibility and providing a basis for future standardization of fatigue testing of additively manufactured lattice structures.