We studied karyological and morphological variability of the only representative of the genus Petrosimonia (Amaranthaceae) in Bulgaria and evaluated the current state of its populations in the country. The results indicated that Bulgarian populations of Petrosimonia brachiata have a diploid chromosome number of 2n = 16. In the population from the Pomorie Lake, the karyotype consists of six pairs of metacentric and two pairs of sub-metacentric chromosomes. In the population from the Atanasovsko Lake, satellites were observed on one pair of sub-metacentric chromosomes. Statistical analysis demonstrated that the main source of phenotype variation in the species is within the populations. Vegetative traits are more variable than generative ones. No significant variability was found in the studied quantitative characteristics or pollen morphology. Given the biological type of the species (annual) and its limited distribution in Bulgaria, the status of the two populations of P. brachiata cannot be regarded as stable, and the danger of a potential threat to them in the future is a possibility not to be excluded.