With increasing thermal footprint in electronic packages, metal lattices offer avenues for better thermal management in passively cooled heat sinks. Combined with the advances in metal additive manufacturing, these can be leveraged to realize complex designs that offer superior heat transfer characteristics with smaller and lighter components. In this work, design, analysis, and optimization of metal lattice-based heat sinks for passive cooling by buoyancy-driven convection is presented. Heat sinks based on three types of lattice structures — Simple Cubic, Voronoi and Octet — have been studied by varying lattice parameters such as thickness and height to obtain the optimal design. Numerical computations using the ANSYS® commercial package have been carried out and the steady state junction temperature at the base of the heat sink monitored. The Voronoi lattice structure provides the best enhancement in heat transfer with ∼18% reduction in junction-to-ambient temperature difference as compared to a standard baseline longitudinal heat sink (LHS), while simultaneously reducing the mass of the heat sink by ∼2.1 times. This behavior is attributed to the enhanced surface area, open-celled lattice structure with lower flow resistance and enhanced thermal dissipation due to local flow disturbances. Based on the junction-to-ambient temperature difference and mass of each heat sink, a Figure of Merit (FOM) is defined to quantify the relative performance of all the designs. The Voronoi lattice at 0.8 mm thickness and 25.4 mm height was found to be the optimal design. It provides the best improvement of ∼3 times compared to the baseline LHS. Also, a Design for Additive Manufacturing (DFAM) approach has been implemented. Virtual build-up simulation indicated suitability of the design for fabrication. The predicted deviation after printing was within the commercially acceptable tolerance limits for use in applications. The workflow presented here from design to analysis, optimization and preparation for fabrication is useful for rapid development of customized passive cooling solutions and determination of optimal designs. We believe that the analysis and methodology presented in this paper would be helpful in the development of better thermal management devices aided by additive manufacturing in future.