This paper presents the first comprehensive checklist of diatoms in Iran, spanning the period from 1842 to 2024. The checklist encompasses 1,073 diatom species, representing 3 phyla, 61 families, 163 genera, and several infraspecific taxa and four endemic species. Records derive from a diverse range of habitats including freshwater, brackish, marine, and saline environments across Iran. Bacillariophyceae show the highest frequency among the diatom taxa, accounting for 87% of the species with 36 families, 119 genera, and 932 species. Mediophyceae ranked second with 8% of the records, represented by 13 families, 25 genera, and 87 species. Coscinodiscophyceae accounted for 5% of the diatom taxa, comprising 12 families, 19 genera, and 54 species. Bacillariaceae were the most diverse family, with 162 species. Furthermore, 17 families were represented by a single genus and species. Phytogeographically, the dominant species were observed in the Hyrcanian and freshwater regions. The highest similarity (87%) was found between the Irano-Torrany and Hyrcanian areas in terms of shared species.