Forty-nine species of organic-cemented siliceous agglutinated foraminifera are described from 11 different facies associations representing basinal deposits of the Triassic in Timor Leste. Among the foraminifera, one genus and five species are named as new: Aaptotoichus mutinensis n. sp., Carteriella manelobasensis n. gen., n. sp., Placentammina soibadaensis n. sp., Subreophax bariquiensis n. sp., and Trochamminopsis manatutoensis n. sp. In overall aspect the fauna has a cosmopolitan composition. The Triassic sediments were probably deposited in rift basins within northern more open parts of the East Gondwana rift system, and are incorporated in the structurally complex Timor orogenic belt. Extensive coherent Triassic stratigraphic sections are not preserved in Timor and stratigraphic reconstruction is based on correlations using conodonts, palynomorphs, and other foraminifera. Organic-cemented siliceous species are recorded from all stages of the Triassic in Timor Leste except the Induan, although most samples come from the Upper Triassic and very few from the Lower Triassic. Time-stratigraphic differentiation of the fauna is not well defined but a local faunal change seems to be represented within the Late Triassic (mid Norian). The facies associations containing the foraminifera range from those influenced by sediment derived from nearby carbonate banks to prodelta and delta-front associations. Faunal differentiation is apparent across this spectrum of facies. A survey of equivalent faunas from selected shallow-marine basins ranging from the Carboniferous to the mid-Cretaceous illustrates the conservative development through time of the siliceous agglutinated foraminifera. Apart from the appearance of large-size unilocular species belonging to short-range genera apparently endemic to a region influenced by meltwater from late Paleozoic Gondwanan ice sheets, these foraminifera show gradual diversification and their stratigraphic record does not seem to be punctuated by abrupt extinctions. This is explained by the adaptability of an estuarine-like fauna living in shallow-marine mud habitats subject to great seasonal variation in water quality and periodic sea-level change. Such a fauna can seek refuge in estuarine environments during low stands of sea level and then repopulate the mud habitats of shallow interior seas as sea level rises. A high degree of adaptability to factors such as changes in salinity, dissolved-oxygen content, temperature, and nutrient influx, along with an infaunal mode of life in variably fluidized mud, may explain why the siliceous agglutinated foraminifera were not affected by major biotic crises during the Permian–Cretaceous interval.