Thomson's gazelle (Eudorcas thomsonii) is one of the best known species of gazelles. It is named after explorer Joseph Thomson and is sometimes referred to as a "tommie". It is considered by some to be a subspecies of the red-fronted gazelle and was formerly considered a member of the genus Gazella within the subgenus Eudorcas, before Eudorcas was elevated to genus status.

Thomson's gazelles can be found in numbers exceeding 200,000 in Africa and are recognized as the most common type of gazelle in East Africa. A small fast antelope, the Thomson's gazelle is said to have top speeds up to 80–90 km/h (50–55 mph). It is the fourth-fastest land animal, after the cheetah (its main predator), pronghorn, and springbok.

The current scientific name of Thomson's gazelle is Eudorcas thomsonii. It is a member of the genus Eudorcas and is classified under the family Bovidae. Thomson's gazelle was first described by British zoologist Albert Günther in 1884. The relationships between Thomson's gazelle and the congeneric Mongalla gazelle (E. albonotata) remain disputed; while some authors such as Alan W. Gentry of the (Natural History Museum, London) consider the Mongalla gazelle to be a subspecies of Thomson's gazelle, others (such as Colin Groves) consider the Mongalla gazelle to be a full species. Zoologist Jonathan Kingdon treated Heuglin's gazelle, sometimes considered a species of Eudorcas (E. tilonura) or a subspecies of the red-fronted gazelle (E. r. tilonura), as a subspecies of Thomson's gazelle. Thomson's gazelle is named after the Scottish explorer Joseph Thomson; the first recorded use of the name dates to 1897. Another common name for the gazelle is "tommy".

Antilope, Eudorcas, Gazella, and Nanger form a clade within their tribe Antilopini. A 1999 phylogenetic analysis showed that Antilope is the closest sister taxon to Gazella, although the earliest phylogeny, proposed in 1976, placed Antilope as sister to Nanger. In a more recent revision of the phylogeny of the Antilopini on the basis of nuclear and mitochondrial data in 2013, Eva Verena Bärmann (of the University of Cambridge) and colleagues constructed a cladogram that clearly depicted the close relationship between Nanger and Eudorcas. Antilope and Gazella were found to have a similar relationship.

Two subspecies are identified:

Thomson's gazelle is a relatively small gazelle; it stands 60–70 cm (24–28 in) at the shoulder. Males weigh 20–35 kg (44–77 lb), while the slightly lighter females weigh 15–25 kg (33–55 lb). Facial characteristics of the gazelle include white rings around the eyes, black stripes running from a corner of the eye to the nose, rufous stripes running from the horns to the nose, a dark patch on the nose, and a light forehead.

The coat is sandy brown to rufous; a black band runs across the flanks, from the upper foreleg to just above the upper hind leg. A buff band occurs above the black stripe. Short, black streaks mark the white rump. The black tail measures 15–27 cm (5.9–10.6 in). Males have well-developed preorbital glands near the eyes, which are used for scent-marking territories. Both sexes possess horns that curve slightly backward with the tips facing forward. The horns, highly ringed, measure 25–43 cm (9.8–16.9 in) in males and 7–15 cm (2.8–5.9 in) in females. However, females have more fragile horns; some are even hornless. Grant's gazelle is very similar to Thomson's gazelle, but can be differentiated by its larger size and the white patch on the rump extending top over the tail.

The two subspecies differ in their appearance. The eastern Thomson's gazelle is the larger of the two, with fainter facial markings. The Serengeti Thomson's gazelle has a whiter face with more conspicuous markings. The horns of females are shorter than those of males to a greater degree in the eastern Thomson's gazelle and the horns are more divergent in the eastern Thomson gazelle.