The Tremp Formation (Spanish: Formación de Tremp, Catalan: Formació de Tremp), alternatively described as Tremp Group (Spanish: Grupo Tremp), is a geological formation in the comarca Pallars Jussà, Lleida, Spain. The formation is restricted to the Tremp or Tremp-Graus Basin (Catalan: Conca de Tremp), a piggyback foreland basin in the Catalonian Pre-Pyrenees. The formation dates to the Maastrichtian to Thanetian, thus the formation includes the Cretaceous-Paleogene boundary that has been well studied in the area, using paleomagnetism and carbon and oxygen isotopes. The formation comprises several lithologies, from sandstone, conglomerates and shales to marls, siltstones, limestones and lignite and gypsum beds and ranges between 250 and 800 metres (820 and 2,620 ft) in thickness. The Tremp Formation was deposited in a continental to marginally marine fluvial-lacustrine environment characterized by estuarine to deltaic settings.

The Tremp Basin evolved into a sedimentary depression with the break-up of Pangea and the spreading of the North American and Eurasian plates in the Early Jurassic. Rifting between Africa and Europe in the Early Cretaceous created the isolated Iberian microplate, where the Tremp Basin was located in the northeastern corner in a back-arc basin tectonic regime. Between the middle Albian and early Cenomanian, a series of pull-apart basins developed, producing a local unconformity in the Tremp Basin. A first phase of tectonic compression commenced in the Cenomanian, lasting until the late Santonian, around 85 Ma, when Iberia started to rotate counterclockwise towards Europe, producing a series of piggyback basins in the southern Pre-Pyrenees. A more tectonically quiet posterior phase provided the Tremp Basin with a shallowing-upward sequence of marine carbonates until the moment of deposition of the Tremp Formation, in the lower section still marginally marine, but becoming more continental and lagoonal towards the top.

Shortly after deposition of the Tremp Formation, the Boixols Thrust, active to the north of the Tremp Basin and represented by the Sant Corneli anticline, started a phase of tectonic inversion, placing upper Santonian rocks on top of the northern Tremp Formation. The main phase of movement of another major thrust fault, the Montsec to the south of the Tremp Basin, happened not before the Early Eocene. Subsequently, the western Tremp Basin was covered by thick layers of conglomerates, creating a purely continental foreland basin, a trend observed going westward in the neighboring foreland basins of Ainsa and Jaca.

A rich and diverse assemblage of fossils has been reported from the formation, among which more than 1000 dinosaur bones, tracks dating up to just 300,000 years before the Cretaceous-Paleogene boundary, and many well-preserved eggs and nesting sites in situ, spread out over an area of 6,000 square metres (65,000 sq ft). Multiple specimens and newly described genera and species of crocodylians, mammals, turtles, lizards, amphibians and fish complete the rich vertebrate faunal assemblage of the Tremp Formation. Additionally, fresh-to-brackish water clams as Corbicula laletana, bivalves of Hippurites castroi, gastropods, plant remains and cyanobacteria as Girvanella were found in the Tremp Formation. The unique paleoenvironment, well-exposed geology, and importance as national heritage has sparked proposals to designate the Tremp Formation and its region as a protected geological site of interest since 2004, much like the Aliaga geological park and others in Spain.

Due to the exposure, the interaction of tectonics and sedimentation and access, the formation is among the best studied stratigraphic units in Europe, with many universities performing geological fieldwork and professional geologists studying the different lithologies of the Tremp Formation. The abundant paleontological finds are displayed in the local natural science museums of Tremp and Isona, where educational programs have been established explaining the geology and paleobiology of the area. In 2016, the Tremp Basin and surrounding areas were filed to become a Global Geopark, and on April 17, 2018, UNESCO accepted this proposal and designated the site Conca de Tremp-Montsec Global Geopark. Spain hosts the second-most Global Geoparks in the world, after China.

The Tremp Formation was defined and named in 1968 by Mey et al., just as the Tremp Basin after the Pre-Pyrenean town of Tremp. The various subdivisions of the formation or alternatively called group, are named after the villages, rivers, canyons and hills in the basin.

The Tremp Formation is a marginally marine to fluvial to lacustrine and continental sedimentary unit with a thickness varying between 250 and 800 metres (820 and 2,620 ft). The formation is found in the Tremp-Graus Basin, a piggyback basin enclosed by the Sant Corneli anticline in the north, the Boixols Thrust in the northeast, the Montsec Thrust in the south and the Collegats Formation in the west. The Tremp-Graus Basin is bordering the Ainsa Basin to the west, and the Àger Basin to the south. The basin is subdivided into four synclinal areas, from east to west Vallcebre, Coll de Nargó, Tremp and Àger. While in Benabarre, the Tremp Formation overlies the Arén Formation, in Fontllonga the formation rests on top of the Les Serres Limestone. The formation is partly laterally equivalent with the Arén Formation. The Tremp Formation is stratigraphically overlain by the late Paleogene, locally called Ilerdiense, Àger Formation and the Alveolina Limestone, though in many parts of the Tremp Basin the formation is exposed and covered by alluvium.

The formation comprises several different lithologies, as sandstones, shales, limestones, marls, lignites, gypsum beds, conglomerates and siltstones have been registered.