Euhelopus is a genus of sauropod dinosaur that lived between 143 and 133 million years ago during the Berriasian and Valanginian ages of the Early Cretaceous in what is now Shandong Province in China. It was a large quadrupedal herbivore. Like sauropods such as brachiosaurids and titanosaurs, Euhelopus had longer forelimbs than hindlimbs. This discovery was paleontologically significant because it represented the first dinosaur scientifically investigated from China: seen in 1913, rediscovered in 1922, excavated in 1923, and studied by T'an during the same year. Unlike most sauropod specimens, it has a relatively complete skull. Euhelopus was a long-necked sauropod similar to Mamenchisaurus, but its affinities are controversial. Most studies favor a close relationship between Euhelopus and titanosaurs, rather than mamenchisaurids.

Since its original description, Euhelopus has often been considered a rather large sauropod. It has been thought to weigh about 15–20 tonnes (17–22 short tons) and attain an adult length of 15 metres (49 ft). Later estimates have downsized this considerably. In 2016, Gregory S. Paul estimated the weight at 3.5 tonnes (3.9 short tons) and the body length at 11 metres (36 ft). Benson et al. estimated its mass at 5.9 tonnes (6.5 short tons), whereas Larramendi et al. estimated it at 3.6 tonnes (4.0 short tons).

Euhelopus was a relatively long-necked sauropod, with a 4-meter neck composed of 17 cervical vertebrae. The presacral vertebrae had a camellate pneumatic structure, made of many small pneumatic chambers, as is characteristic of titanosauriforms and some mamenchisaurids. Pneumatic chambers even extended into the ilium, as occurs in many titanosaurs. The humerus was apparently nearly as long as the femur, although there is some uncertainty in this ratio, as it is not certain that the humerus and femur from which this ratio was calculated belong to the same individual, and the femur in question is incomplete.

The original diagnosis by Wiman is outdated. A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some of the features in a diagnosis may be autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to an organism or group.

According to a study by Jeffrey A. Wilson and Paul Upchurch in 2009, Euhelopus can be distinguished based on, among others, these autapomorphies:

The original discovery was by a Catholic priest, Father R. Mertens, in 1913. He showed some remains he had excavated to the German mining engineer Gustav Behaghel who in 1916 sent three vertebrae to the head of the Geological Survey of China Ding Wenjiang ("V.K. Ting"). This was probably the first occasion dinosaur bones from China were scientifically studied. With help of another Catholic priest, Father Alfred Kaschel, the site was rediscovered in November 1922 by Johan Gunnar Andersson and Tan Xichou. In March 1923, the Austrian student Otto Zdansky excavated two skeletons at sites about three kilometres apart and the holotype was studied by H. C. T'an, also in 1923.

It was originally described and named Helopus, meaning "Marsh Foot", by the Swedish paleontologist Carl Wiman in 1929, after the Greek ἕλος, helos, "swamp", and πούς, pous, "foot". The name refers to the marshy area of the finds and to truga, Swedish swamp shoes, which according to Wiman resembled the wide feet of the animal. This name however, already belonged to a bird because the Caspian tern had once been named Helopus caspius Wagler 1832. The sauropod dinosaur was therefore renamed Euhelopus (True marsh-foot) in 1956 by Alfred Sherwood Romer. There proved to be a plant genus (a grass) with the same generic name, Euhelopus. However, a genus in one biological kingdom may have a name that is used as a genus name in another kingdom, so Euhelopus was allowed. The type species is Helopus zdanskyi. The combinatio nova is Euhelopus zdanskyi. The specific name honours Zdansky.

Specimen PMU 24705 (formerly PMU R233) forms according to Wilson & Upchurch the holotype, descriptive basis, for the species Euhelopus zdanskyi. It represents one of the skeletons found by Zdansky, named "Exemplar a" by Wiman, who did not formally assign a holotype. This is the original skeleton found by Mertens. Specimen PMU 24705 consists of a partial skeleton with skull and lower jaws comprising these bones: the rostral part of the left nasal; a partial right jugal; the tapered jugal process of the postorbital, partially excavated; the dorsal process of the right quadratojugal; the fragmented left pterygoid (another fragment might be the right splenial, but it is too fragile to be removed from its matrix), a series of twenty-five presacral vertebrae and the left thighbone.