Virus like particles, and other naturally occurring protein containers, have emerged as excellent building blocks for nanomaterials design and synthesis. Here we exploit a directed assembly and encapsulation approach to sequester multiple copies of a phosphotriesterase (PTE) enzyme within the capsid of bacteriophage P22. Phosphotriesterase, from Brevundimonas diminuta, is an intriguing enzyme as it is highly active against a wide range of harmful insecticides and nerve agents such as Soman and Sarin. However, difficulty in expressing large quantities of the active recombinant enzyme has limited efforts to scale-up its use. Additionally, as a mesophilic enzyme its low heat tolerance and susceptibility to proteolysis makes it a less than ideal candidate as a practical bioremediation tool. Through encapsulation of the PTE within the P22 capsid, we demonstrate a greatly enhanced thermal tolerance of the enzyme, maintaining 50% of its activity to 60 °C. Additionally, the P22 capsid confers protection to the enzyme from proteases, as well as stabilizing the enzyme against desiccation. Thus, our engineered P22 encapsulation system greatly enhances the stability of the mesophilic phosphotriesterase and results in a robust and active nanoparticle reactor.