The English Mastiff, or simply the Mastiff, is a British breed of very large dog of mastiff type. It is likely descended from the ancient Alaunt and Pugnax Britanniae, with a significant input from the Alpine Mastiff in the 19th century. Distinguished by its enormous size, massive head, short coat in a limited range of colours, and always displaying a black mask, the Mastiff is noted for its gentle and loving nature. The lineage of modern dogs can be traced back to the early 19th century, but the modern type was stabilised in the 1880s and refined since. Following a period of sharp decline, the Mastiff has increased its worldwide popularity, although it continues to be classed as a vulnerable breed in its country of origin, with only 104 registered births recorded in the UK in 2020. Throughout its history the Mastiff has contributed to the development of a number of dog breeds, some generally known as mastiff-type dogs or, confusingly, just as "mastiffs". It is the largest living canine, outweighing the wolf by up to 50 kg (110 lbs) on average.

There is a ceramic and paint sculpture of a mastiff-like dog from Mesopotamia region during the Kassite period (mid-2nd millennium B.C.).

These dogs may be related to the dogs that fought lions, tigers, bears, and gladiators in Roman arenas. Certainly an element in the formation of the English Mastiff was the Pugnax Britanniae that existed at the time of the Roman conquest of Britain. The ancient Roman poet Grattius (or Grattius Faliscus) wrote of British dogs, describing them as superior to the ancient Greek Molossus, saying:

What if you choose to penetrate even among the Britons? How great your reward, how great your gain beyond any outlays! If you are not bent on looks and deceptive graces (this is the one defect of the British whelps), at any rate when serious work has come, when bravery must be shown, and the impetuous War-god calls in the utmost hazard, then you could not admire the renowned Molossians so much.

The turn-of-the-millennium Greek historian Strabo reported that dogs were exported from Britain for the purpose of game hunting, and that these dogs were also used by the Celts as war dogs. As far as the origin of the Pugnax Britanniae is concerned, there is unproven speculation that they were descended from dogs brought to Britain by the Phoenicians in the 6th century BC. This breed's first written accounts in England date back to 55 BC when Caesar noted them during his invasions. Many were sent to Italy and in the Roman Empire they became fighting dogs.

The Alaunt is likely to have been another genetic predecessor to the English Mastiff. Introduced by the Normans, these dogs were developed by the Alans, who had migrated into France (then known as Gaul) due to pressure by the Huns at the start of the 5th century. Intriguingly they were known from the Romans to live in a region (the Pontic-Caspian Steppe) about 700 km to the north of the region where the Assyrians once lived. Again, any canine connections are speculative.

The linguistic origin of the name "Mastiff" is unclear. Many claim that it evolved from the Anglo-Saxon word "masty", meaning "powerful". Other sources, such as the Oxford English Dictionary, say the word originated from the Old French word mastin (Modern French mâtin), the word being itself derived from Vulgar Latin *ma(n)suetinus "tame", see Classical Latin mansuetus with same meaning. The first list of dog breed names in the English language, contained within The Book of Saint Albans, published in 1465, includes "Mastiff ". This work is attributed to Prioress Juliana Berners, but in part may be translated from the early 14th century Norman-French work Le Art de Venerie, by Edward II's Huntmaster Guillaume Twici. In 1570, Conrad Heresbach, in Rei Rusticae Libri Quatuor, referred to "the Mastie that keepeth the house". Heresbach was writing in Latin; his work was translated a few years later into English by Barnabe Googe as Foure Bookes of Husbandrie. This work was originally adapted from De Re Rustica by 1st century Roman writer Columella, which highlights the Roman connection. Certainly from Roman to medieval times, Mastiff-like dogs were used in the blood sports of bull-baiting, bear-baiting, lion-baiting, and dog fighting, as well as for hunting and guarding. Dogs known as Bandogs, who were tied (bound) close to houses, were of the Mastiff type. They were described by John Caius in 1570 as vast, huge, stubborn, ugly, and eager, of a heavy and burdensome body—noted for their use as guard dogs on remote estates.

The naturalist Christopher Merret in his 1666 work Pinax Rerum Naturalium Brittanicarum has a list of British mammals, including 15 kinds of dog, one of which is "Molossus, Canis bellicosus Anglicus, a Mastif". Literally, "Molossus, warlike English dog, a Mastiff", and perhaps the first conflation of the breeds Molossus and Mastiff.