A class of ultralight, ultrastiff mechanical metamaterials has been developed, maintaining nearly constant stiffness per unit mass density across a wide range of densities. These materials, composed of micro-architected structures with high structural connectivity and nanoscale features, are fabricated using projection microstereolithography combined with nanoscale coating and post-processing. The mechanical properties of these materials are defined by their geometry rather than their composition, and they exhibit a linear scaling relationship between stiffness and density over three orders of magnitude. This is achieved through a stretch-dominated unit cell structure with high nodal connectivity and a face-centered cubic architecture, which allows for efficient load transfer in tension or compression. The materials are fabricated with constituent materials ranging from polymers to metals and ceramics, and include both solid and hollow-tube structures. The results show that these materials have a significantly higher stiffness-to-weight ratio compared to conventional materials, and they maintain their mechanical efficiency over a broad density range. The study also highlights the importance of microstructure in achieving mechanical performance, and demonstrates the potential of these materials for a wide range of applications, including structural components, energy absorption, heat exchange, catalyst supports, filtration, and biomaterials.A class of ultralight, ultrastiff mechanical metamaterials has been developed, maintaining nearly constant stiffness per unit mass density across a wide range of densities. These materials, composed of micro-architected structures with high structural connectivity and nanoscale features, are fabricated using projection microstereolithography combined with nanoscale coating and post-processing. The mechanical properties of these materials are defined by their geometry rather than their composition, and they exhibit a linear scaling relationship between stiffness and density over three orders of magnitude. This is achieved through a stretch-dominated unit cell structure with high nodal connectivity and a face-centered cubic architecture, which allows for efficient load transfer in tension or compression. The materials are fabricated with constituent materials ranging from polymers to metals and ceramics, and include both solid and hollow-tube structures. The results show that these materials have a significantly higher stiffness-to-weight ratio compared to conventional materials, and they maintain their mechanical efficiency over a broad density range. The study also highlights the importance of microstructure in achieving mechanical performance, and demonstrates the potential of these materials for a wide range of applications, including structural components, energy absorption, heat exchange, catalyst supports, filtration, and biomaterials.