A new species of the ‘flat-backed’ millipede family Polydesmidae, Propolydesmus cretaceus sp. nov., is described from the mid-Cretaceous Burmese amber. Detailed morphological characters are provided on the basis of 3 well-preserved adults (2 males and 1 females), 2 female subadults, and 14 additional specimens, using confocal laser scanning microscopy (CLSM) and X-ray micro-computed tomography (μCT) with computer-aided 3D-reconstructions. The new species can be placed in the extant genus Propolydesmus. It majorly differs from other congeners due to the stronger gonopod acropodite and the presence of a bipartite gonopod exomerite.