None of the 96 cases with other ACTB pathogenic variants in the Human Gene Mutation Database had diabetes.<h4>Interpretation</h4>The identification of 3 individuals (two in this report and one from the literature) with neonatal diabetes and a de novo ACTB p.(Ser348Leu) variant supports ACTB as a previously unrecognised neonatal diabetes aetiological gene, most likely through a variant-specific mechanism.<h4>Funding</h4>Diabetes UK; EFSD/NNF; NIHR, Wellcome Trust. Here, ACTB is linked to diabetes mellitus.