Nature provides powerful but as-yet largely unharnessed methods for low-cost, green synthesis of inorganic functional materials such as quantum dots. These materials have diverse applications from medicine to renewable energy. Harnessing nature’s unique ability to achieve cost effective and scalable manufacturing solutions with reduced environmental impact is integral to realizing a future biomanufacturing economy. To address this challenge, a bacterial strain has been engineered to enable biosynthesis of CdS nanocrystals with extrinsic crystallite size control in the quantum confinement range. This strain yields extracellular, water-soluble quantum dots from low-cost precursors at ambient temperatures and pressure. The biomanufacturing approach demonstrated here produces CdS semiconductor nanocrystals with associated size-dependent band gap and photoluminescent properties.