Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from every continent. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thriving at the time of the extinction event at the end of the Cretaceous. This group includes some of the largest land animals known to have ever existed, such as Patagotitan, estimated at 37 m (121 ft) long with a mass of 69 tonnes (76 tons), and the comparably-sized Argentinosaurus and Puertasaurus from the same region.

The group's name alludes to the mythological Titans of ancient Greek mythology, via the type genus (now considered a nomen dubium) Titanosaurus. Together with the brachiosaurids and relatives, titanosaurs make up the larger sauropod clade Titanosauriformes. Titanosaurs have long been a poorly-known group, and the relationships between titanosaur species are still not well-understood.

Due to the near-global distribution of titanosaurs during the Cretaceous, titanosaur fossils have been found on every continent, including Antarctica. However, titanosaurs have the least complete fossil record of any major sauropodomorph group. No complete titanosaur skeletons are known, and many species are only known from a few bones. Titanosaur skulls are especially rare. Though fragmentary cranial remains are known for several titanosaur genera, nearly complete skulls have been described for only four: Nemegtosaurus, Rapetosaurus, Sarmientosaurus, and Tapuiasaurus. As is the case in most other sauropod groups, there are few titanosaur specimens with complete necks preserving all of the cervical vertebrae in sequence.

Only three complete titanosaur necks are known: the holotype of Futalognkosaurus and two undescribed specimens from Argentina. A fourth specimen, of an unidentified titanosaur from Brazil, preserves a nearly complete neck, with only the atlas, the tiny vertebra forming the joint between the skull and neck, missing. Only five titanosaur specimens preserve complete, articulated hind feet. This incompleteness is especially significant for giant titanosaurs, which are generally known from disarticulated and fragmentary remains.

Titanosaurs are one of the few groups of dinosaurs for which fossil eggs are known. The fossil site of Auca Mahuevo preserves a titanosaur nesting ground. Some titanosaur eggs have been found containing fossil embryos, which even preserve fossil skin. These fossil embryos are among the few titanosaur specimens to preserve complete skulls.

Titanosauria have the largest range of body size of any sauropod clade, and includes both the largest known sauropods and some of the smallest. One of the largest titanosaurs, Patagotitan, had a body mass estimated to be 69 tonnes (76 tons), whereas one of the smallest, Magyarosaurus, had a body mass of approximately 900 kilograms (2,000 lb). Even relatively closely related titanosaurs could have very different body sizes, as the small rinconsaurs were closely related to the gigantic lognkosaurs. Fossils from perhaps the largest dinosaur ever found were discovered in 2021 in the Neuquén Province of northwest Patagonia, Argentina. It is believed that they are from a titanosaur. Some of smallest titanosaurs, such as Magyarosaurus, inhabited Europe, which was largely made up of islands during the Cretaceous, and were likely island dwarfs. Another taxon of tiny titanosaurs, Ibirania, lived a non-insular context in Upper Creaceous Brazil, and is an example of nanism resultant from other ecological pressures.

The heads of titanosaurs are poorly known. However, several different cranial morphologies are apparent. In some species, such as Sarmientosaurus, the head resembled that of brachiosaurids. In others, such as Rapetosaurus and Nemegtosaurus, the head resembled that of diplodocids. In some titanosaurs, the skull was especially diplodocid-like due to square-shaped jaws; the titanosaur Antarctosaurus is especially similar to the rebbachisaurid Nigersaurus. Titanosaurs had small heads, even when compared with other sauropods. The head was also wide, similar to the heads of Camarasaurus and Brachiosaurus, though somewhat more elongated. Titanosaurian nostrils were large ("macronarian") and all had crests formed by the nasal bones. Their teeth were either somewhat spatulate (spoon-like) or like pegs or pencils, but were always very small.

Titanosaur necks were of average length for sauropods, and their tails were whip-like though not as long as in the diplodocids. While the pelvis was slimmer than some sauropods, the pectoral (chest) area was much wider, giving them a uniquely "wide-legged" stance. As a result, the fossilized trackways of titanosaurs are distinctly broader than other sauropods. Their forelimbs were also stocky, and often longer than their hind limbs. Unlike other sauropods, some titanosaurs had no digits, walking only on horseshoe-shaped "stumps" made up of the columnar metacarpal bones. Their vertebrae (back bones) were solid (not hollowed-out), which may be a reversal to more basal saurischian characteristics. Their spinal column was relatively flexible, likely making them more agile than other sauropods, though at the expense of rearing on their hind legs compared to the Diplodocoids. One of the most characteristic features shared by most titanosaurs were their procoelous caudal vertebrae, with ball-and-socket articulations between the vertebral centra.