The Barremian–Aptian sediments exposed in the Jozak stratigraphic section, west of Bojnourd in northeastern Iran, are mainly composed of alternating thin- to thick-bedded limestones belonging to the Tirgan Formation. Detailed micropaleontological investigations of the studied succession revealed a rich assemblage of calcareous algae and benthic foraminifera, particularly orbitolinids, which are important biostratigraphic markers for Lower Cretaceous carbonate platform deposits. The identified microfossil assemblage provides valuable information on the age, depositional environment, and paleogeographic significance of the Tirgan Formation in the study area and allows a more precise regional biostratigraphic correlation with equivalent Lower Cretaceous successions in adjacent areas.Among the recognized taxa, several poorly known species of the genus Orbitolinopsis, including Orbitolinopsis buccifer, O. cuvillieri, and O. kiliani, are recorded for the first time from the Tirgan Formation in northeastern Iran. The occurrence of these species, together with other characteristic benthic foraminifera, provides strong evidence for assigning a Late Barremian–Early Aptian age to the studied interval. In addition, the abundance and diversity of orbitolinids indicate deposition within a shallow-marine carbonate platform environment under warm-water conditions. The results of this study improve the biostratigraphic framework of the Tirgan Formation and contribute to a better understanding of the distribution, evolution, paleoecology, and paleobiogeographic relationships of Lower Cretaceous orbitolinid assemblages in the eastern Alborz and Kopet-Dagh sedimentary basins.