Nanoparticle technologies provide a powerful tool for the development of reagents for use in both therapeutic and diagnostic, or theragnostic biomedical applications. Two broad classes of particles are under development, viral and synthetic systems, each with their respective strengths and limitations. Here we adapt the phage lambda system to construct modular designer nanoparticles that blend these two approaches. We have constructed a variety of modified decoration proteins that allow site-specific modification of the shell with both protein and nonproteinaceous ligands including small molecules, carbohydrates, and synthetic display ligands. We show that the chimeric proteins can be used to simultaneously decorate the shell in a tunable surface density to afford particles that are physically homogeneous and that can be manufactured to display a variety of ligands in a defined composition. These designer nanoparticles set the stage for development of lambda as a theragnostic nanoparticle system.