Iron hydroxide adipate tartrate (IHAT) is a nano-scale material with a diameter of less than 5 nm intended for use as iron source in food supplements. It is the first engineered nanomaterial that was authorised as novel food in the European Union in 2022 (Commission Implementing Regulation (EU) 2022/1373) and subsequently added to the European Union list of novel foods. Official control laboratories of EU Member States are required to conduct compliance testing of novel foods, including IHAT, against approved specifications. In the case of IHAT, the authorised specifications include the constituent particle size, referred to in the implementing regulation as ‘primary particle size’. In this study, a method to determine the constituent particle size distribution of IHAT using the volume-based hydrodynamic diameter with dynamic light scattering is proposed and tested in an interlaboratory comparison study involving five experienced European food control and food research laboratories. Three test materials were examined, including an early batch of IHAT without the flow agent, a damaged material unintentionally exposed to water, and the final commercial product that included the flow agent. The results demonstrate that dynamic light scattering can reliably measure the particle size of particles in the low nanometre range if samples are prepared and measured following a robust, easy-to-reproduce, and standardised protocol. The results obtained for the commercial product met the compliance requirements specified in the annex of the Commission Implementing Regulation in all laboratories for the primary particle size distribution (Dv(10/50/90)). The method proposed in this study could represent an initial step towards the full validation of the method.