Eolambia (meaning "dawn lambeosaurine") is a genus of herbivorous hadrosauroid dinosaur from the early Late Cretaceous of the United States. It contains a single species, E. caroljonesa, named by paleontologist James Kirkland in 1998. The type specimen of Eolambia was discovered by Carole and Ramal Jones in 1993; the species name honors Carole. Since then, hundreds of bones have been discovered from both adults and juveniles, representing nearly every element of the skeleton. All of the specimens have thus far been found in Emery County, Utah, in a layer of rock known as the Mussentuchit Member of the Cedar Mountain Formation.

Measuring up to 6 meters (20 ft) long, Eolambia is a large member of its group. While it closely approaches the Asian hadrosauroids Equijubus, Probactrosaurus, and Choyrodon, in traits of the skull, vertebrae, and limbs, it may actually be more closely related to the North American Protohadros. This grouping, based on the straightness of the quadrate bone and scapula, would represent an isolated, endemic radiation of hadrosauroids. Despite resembling hadrosaurids – lambeosaurine hadrosaurids in particular – in several features, leading to its initial identification as one of them, these similarities have been rejected as either entirely convergent or misinterpreted.

Eolambia would have lived in a forested environment at the edge of lakes in a humid floodplain environment, feeding on gymnosperms, ferns, and flowering plants. The water levels in the lakes changed over time with cyclical wet and dry spells caused by the precession of the Earth, reflected by alternating bands in the sediments of the Mussentuchit Member. As a juvenile, Eolambia would have been preyed upon by large crocodylomorphs residing in the lake waters. With increasing age, however, they became impervious to the crocodylomorphs, and mature individuals (at least eight to nine years in age) were preyed on by large theropods such as the neovenatorid Siats.

In 1979 Peter Galton and James A. Jensen described a fragmentary right femur, BYU 2000, belonging to a hadrosaurian dinosaur discovered in sediments belonging to the Cedar Mountain Formation in Arches National Park, Utah. Though poor material, it was important for it (alongside a second North American femur described in the paper) was the first hadrosaur specimen from the Lower Cretaceous anywhere in the world. Galton and Jensen hypothesized more complete remains of a hadrosaur may be found from the formation in the future. Various hadrosauroid teeth had also been found in quarries of small vertebrates in the western region of the San Rafael Swell, near Castle Dale in Emery County, Utah; they were described in 1991 by J. Michael Parrish. Subsequently, in 1993, Carole Jones and her husband Ramal Jones discovered fragmentary bones in a fossil site located in the northwestern region of the Swell. They brought the site to the attention of Donald Burge, director of the institution then called the College of Eastern Utah Prehistoric Museum (CEUM). The site, which is formally known as CEUM Locality 42em366v, would subsequently be named Carol's Site (sic) in her honour. The fossils, stored under the specimen number CEUM 9758, represent the partial remains of an adult hadrosauroid, including parts of the skull, vertebrae, ischium, and leg. CEUM 5212, a partial skull and forelimb from an adult, was found nearby in CEUM Locality 42em369v. CEUM 8786, a left femur from an adult, was discovered later in Carol's Site, and was not described until 2012.

At the same time, the Oklahoma Museum of Natural History (OMNH) had recovered remains belonging to the same hadrosauroid from excavations in the southwestern region of the Swell. Specimens discovered by the OMNH initially consisted of six individuals from five localities: two juvenile skeletons, including vertebrae, scapulae, an ulna, an ilium and ischium, and hindlimb elements from locality OMNH v237; a partial juvenile skeleton, including parts of the skull, forelimbs, and hindlimbs along with a dorsal vertebra from OMNH v824; OMNH 27749, a sacrum and ischium from OMNH v696; OMNH 24389, an ischium from OMNH v214; and OMNH 32812, a partial skeleton including a scapula, two caudal vertebrae, and other unexcavated elements from OMNH v866. The OMNH also made further discoveries of the hadrosauroid's teeth in various quarries. Richard Cifelli of the OMNH allowed James Kirkland to study the collected specimens. Kirkland went on to preliminarily describe the specimens in 1994 in a conference abstract at the annual meeting of the Society of Vertebrate Paleontology.

In a 1998 paper published in the bulletin of the New Mexico Museum of Natural History and Science, Kirkland formally named the specimens as a new genus, Eolambia, with the type and only species being Eolambia caroljonesa. The generic name combines the Greek prefix eos/ἠώς ("dawn", "morning", implying "early") with the suffix lambia (derived from Lambeosaurus, which is in turn named after Canadian paleontologist Lawrence Lambe). In all, the genus name means "dawn (or early) lambeosaurine", in reference to its supposed position as the most basal lambeosaurine. Meanwhile, the specific name honors Carole Jones. The name Eolambia was suggested by paleoartist Michael Skrepnick; it replaced the informal name "Eohadrosaurus caroljonesi", which was used by Kirkland before the 1998 paper. However, the epithet caroljonesa, following the ICZN, is incorrect and in this case the correct spelling would be caroljonesae.

Additional excavations since 1998 have revealed additional skeletal elements belonging to Eolambia. It is now the most completely-known iguanodont from between the Cenomanian and Santonian stages. Nearly every single skeletal element belonging to the taxon is represented by multiple fossils across existing collections. At least twelve individuals, including at least eight juveniles, were discovered in a quarry south of the town of Emery, including parts from both the skull and the rest of the body. The quarry is known as the Cifelli #2 ''Eolambia caroljonesa'' Quarry, or more formally CEUM locality 42em432v. Another quarry south of the town of Emery – the Willow Springs 8 quarry, or CEUM locality 42Em576v – preserves at least four juveniles. Material from these quarries was described in 2012 by Kirkland along with Andrew McDonald, John Bird, and Peter Dodson, who also provided an updated diagnosis for Eolambia based on this material.

One specimen discovered at the Cifelli #2 quarry, a right dentary catalogued as CEUM 34447, is unusual compared to other Eolambia specimens, including both adults and juveniles. While the ratio between the mid-point depth and overall length of the dentaries in other individuals generally ranges from 0.19 to 0.24, the same value in CEUM 34447 is 0.31, making it unusually deep. The dentary also does not deepen substantially at the front end, and thus lacks one of the distinguishing traits of Eolambia dentaries. However, the other distinguishing trait, which is the expansion of the front end of the coronoid process, is present in the specimen. Although it is possible that this individual represents an unusual Eolambia specimen, McDonald and colleagues cautioned that it may represent a distinct hadrosauroid that differs in the morphology of the dentary from Eolambia (albeit one that is not suggested by any of the other materials at the quarry). Thus, they regarded it as an indeterminate basal hadrosauroid.