Giraffatitan (name meaning "titanic giraffe") is a genus of sauropod dinosaur that lived during the late Jurassic Period (Kimmeridgian–Tithonian stages) in what is now Lindi Region, Tanzania. Only one species is known, G. brancai, named in honor of German paleontologist Wilhelm von Branca, who was a driving force behind the expedition that discovered it in the Tendaguru Formation. Giraffatitan brancai was originally described by German paleontologist Werner Janensch as a species of the North American sauropod Brachiosaurus from the Morrison Formation, as Brachiosaurus brancai. Recent research shows that the differences between the type species of Brachiosaurus and the Tendaguru material are so large that the African material should be placed in a separate genus.

Giraffatitan was for many decades known as the largest dinosaur but recent discoveries of several larger dinosaurs prove otherwise; giant titanosaurians appear to have surpassed Giraffatitan in terms of sheer mass. Also, the sauropod dinosaur Sauroposeidon is estimated to be taller and possibly heavier than Giraffatitan. Most size estimates for Giraffatitan are based on the specimen HMN SII, a subadult individual, but there is evidence supporting that these animals could grow larger; specimen HMN XV2, represented by a fibula 13% larger than the corresponding material on HMN SII, would have measured around 23–26 metres (75–85 ft) long and weighed about 40–48 metric tons (44–53 short tons).

In 1906, mining engineer Bernhard Wilhelm Sattler, while travelling, noticed an enormous bone jutting out of the ground at the Tendaguru (the "steep hill") near Lindi, in what was then German East Africa, today Lindi Region, Tanzania. In early 1907, his superior Wilhelm Arning in Hannover received a report on the find. Arning again informed the Kommission für die landeskundliche Erforschung der Schutzgebiete, a commission in Berlin overviewing the geographical investigation of German protectorates. The German secretary of state of colonies, Berhard Dernburg, at the time visited German East Africa accompanied by the industrialist Heinrich Otto. Otto had invited the paleontologist Professor Eberhard Fraas to join him as a scientific advisor. In the summer of 1907, Fraas, already for some months travelling the colony, received a letter from Dr Hans Meyer in Leipzig, urging him to investigate Sattler's discovery. On 30 August, Fraas arrived by steamer at the coastal town of Lindi. A five-day march brought him to the Tendaguru, where he could confirm that the bones were authentic and dinosaurian. Soon Sattler joined him with a team of native miners who uncovered two large sauropod skeletons which were transported to Germany. Ultimately, these would become the holotypes of the genera Tornieria and Janenschia.^{[citation needed]}

Fraas had observed that the Tendaguru layers were exceptionally rich in fossils. After his return to Germany, he tried to raise enough money for a major expedition. He managed to attract the interest of Professor Wilhelm von Branca, the head of the Geologisch-Paläontologische Institut und Museum der Königliche Friedrich-Wilhelm Universität zu Berlin. Von Branca considered it a matter of German national pride that such a project would succeed. He involved the well-connected pathologist David von Hansemann. Von Hansemann founded a Tendaguru Committee headed by Johann Albrecht, the Duke of Mecklenburg. Soon it became fashionable to join this committee which counted a large number of prominent German industrialists and scientists among its members. Many of their wealthy friends donated considerable sums. To lead the expedition, von Branca sent out one of his curators, Werner Janensch, and one of his assistants, Edwin Hennig. Both men arrived in Dar es Salaam on 2 April 1909.

The expedition initially employed about 160 native porters, as beasts of burden could not be used because of the danger posed by the tse tse fly. During four field seasons, of 1909, 1910, 1911 and 1912, about a hundred paleontological quarries were opened. Large amounts of fossil material were shipped to Germany. Soon it became evident that apart from Tornieria and Janenschia, other sauropods were present in the layers. One was the medium-sized Dicraeosaurus, a relatively common find. More rare was a gigantic form that far surpassed the others in magnitude and that is today known as Giraffatitan. The first quarry with Giraffatitan material was "Site D", located about 1 km (0.62 mi) northeast of the Tendaguru Hill and opened on 21 June 1909. It contained a relatively complete skeleton of a medium-sized individual, lacking the hands, the neck, the back vertebrae and the skull. It included an articulated series of 29 tail vertebrae. The other bones were found in close association on a surface of 22 m² (240 sq ft). "Site IX", located 1.4 km (0.87 mi) northeast of the Tendaguru Hill, was opened on 17 August 1909. Among an assemblage of 150 disarticulated dinosaur bones, also two Giraffatitan thighbones were present. The next Giraffatitan quarry was "Site N", at 900 m (0.56 mi) east of the Tendaguru Hill, excavated in September 1909. It held a single disarticulated skeleton containing a back vertebra, a tail vertebra, ribs, a scapula, a possible scapula, a humerus, two ischia and a number of unidentifiable bones.

The most important source of Giraffatitan fossils would be "Site S" at one kilometre southwest of the hill. Excavations started on 11 October 1909 and continued well into 1912. In 1909 limb and girdle elements were dug up. During 1910, a cut bank of the Kitukituki river was gradually deepened, removing a high overburden. To prevent the quarry walls from collapsing, they were covered by a high wooden framework. That year, first several ribs were uncovered and later part of the vertebral column. In October, close to some neck vertebrae a skull and lower jaws were discovered. From 5 June 1912 onwards more neck and trunk vertebrae were found. Initially it was thought that a single skeleton was being uncovered. Only much later Janensch realised that two skeletons had been present. Skelett SI was represented by a skull, six neck vertebrae and some back vertebrae. Skelett SII was larger but despite its size still a subadult individual. It included skull bones, a series of eleven neck and eleven back vertebrae, ribs, the left scapula, both coracoids, both forelimbs, the pubic bones and the right hindlimb. The sacrum and the tail had been lost to relatively recent erosion. The animal was found in an upright position with vertical limbs, which has been explained by its becoming mired in mud.

In early October 1909, "Site ab" was excavated, 1.2 km (0.75 mi) northeast of the hill. Among disarticulated remains of many sauropods, also two Giraffatitan thighbones were collected. A gigantic possible humerus was too damaged to be salvaged. "Site cc", 2.9 km (1.8 mi) northeast of the hill, contained a disarticulated Giraffatitan skeleton including neck vertebrae, a trunk vertebra, ribs, a scapula and a humerus. In 1910, another Giraffatitan quarry was opened, "Site Y" at 3.1 km (1.9 mi) north of the Tendaguru Hill. It contained the skeleton of a medium-sized individual including a braincase, a series of eight neck vertebrae, a trunk vertebra, ribs, both scapulae, a coracoid, a left humerus and a left fibula.

A partial skeleton called "Skelett Y" (skeleton Y) was discovered in quarry "Y". On the basis of its scapula (shoulder blade) and humerus (upper arm bone) Brachiosaurus fraasi was erected, which later turned out to be a junior synonym of B. brancai. From quarry "D" came 23 vertebrae articulated with a sacrum (a hip bone), from quarry "Aa" another 18 vertebrae with a sacrum, from quarry "no" 50 articulated tail vertebrae, from GI 16 tail vertebrae and a number of limb bones, and much more material from other places, including many limb elements.^{[citation needed]}