The development of a scalable photochemical flow process towards spherical gold nanoparticles is reported. This protocol relies on a water-soluble acylphosphinate photoinitiator that undergoes Norrish type I cleavage upon exposure to UV irradiation and produces radicals able to reduce chloroauric acid into Au(0), leading to the formation of spherical nanoparticles within a short irradiation time (10 s). Robust, reproducible and scalable conditions are optimized for ensuring stable production campaigns under flow conditions. The optimized protocol includes a phosphate buffer to temper pH variations, an external stabilizer (sodium citrate), and finely tuned process conditions. This protocol precludes the classical formation of a gold coating on the internal wall of the reactor, hence alleviating one of the most commonly described obstacles for the development of scalable and robust processes towards gold nanoparticles. The process also features the implementation of an in-line UV spectrometer for real-time process monitoring. Upon optimization, the conditions are successfully transposed to pilot and production scales using commercial mesofluidic photoreactors. An extrapolated productivity of up to 360 L per day of aqueous colloidal spherical gold nanoparticles (corresponding to an isolated yield of 65 g per day) is achieved.