Canis Major is a constellation in the southern celestial hemisphere. In the second century, it was included in Ptolemy's 48 constellations, and is counted among the 88 modern constellations. Its name is Latin for "greater dog" in contrast to Canis Minor, the "lesser dog"; both figures are commonly represented as following the constellation of Orion the hunter through the sky. The Milky Way passes through Canis Major and several open clusters lie within its borders, most notably M41.

Canis Major contains Sirius, the brightest star in the night sky, known as the "dog star". It is bright because of its proximity to the Solar System and its intrinsic brightness. In contrast, the other bright stars of the constellation are stars of great distance and high luminosity. At magnitude 1.5, Epsilon Canis Majoris (Adhara) is the second-brightest star of the constellation and the brightest source of extreme ultraviolet radiation in the night sky. Next in brightness are the yellow-white supergiant Delta (Wezen) at 1.8, the blue-white giant Beta (Mirzam) at 2.0, blue-white supergiants Eta (Aludra) at 2.4 and Omicron² at 3.0, and white spectroscopic binary Zeta (Furud), also at 3.0. The red hypergiant VY CMa is one of the largest stars known, while the neutron star RX J0720.4-3125 has a radius of a mere 5 km.

In ancient Mesopotamia, Sirius, named KAK.SI.SA₂ by the Babylonians, was seen as an arrow aiming towards Orion, while the southern stars of Canis Major and a part of Puppis were viewed as a bow, named BAN in the Three Stars Each tablets, dating to around 1100 BC. In the later compendium of Babylonian astronomy and astrology titled MUL.APIN, the arrow, Sirius, was also linked with the warrior Ninurta, and the bow with Ishtar, daughter of Enlil. Ninurta was linked to the later deity Marduk, who was said to have slain the ocean goddess Tiamat with a great bow, and worshipped as the principal deity in Babylon. The Ancient Greeks replaced the bow and arrow depiction with that of a dog.

In Greek Mythology, Canis Major represented the dog Laelaps, a gift from Zeus to Europa; or sometimes the hound of Procris, Diana's nymph; or the one given by Aurora to Cephalus, so famed for its speed that Zeus elevated it to the sky. It was also considered to represent one of Orion's hunting dogs, pursuing Lepus the Hare or helping Orion fight Taurus the Bull; and is referred to in this way by Aratos, Homer and Hesiod. The ancient Greeks refer only to one dog, but by Roman times, Canis Minor appears as Orion's second dog. Alternative names include Canis Sequens and Canis Alter. Canis Syrius was the name used in the 1521 Alfonsine tables.

The Roman myth refers to Canis Major as Custos Europae, the dog guarding Europa but failing to prevent her abduction by Jupiter in the form of a bull, and as Janitor Lethaeus, "the watchdog". In medieval Arab astronomy, the constellation became al-Kalb al-Akbar, "the Greater Dog", transcribed as Alcheleb Alachbar by 17th century writer Edmund Chilmead. Islamic scholar Abū Rayḥān al-Bīrūnī referred to Orion as Kalb al-Jabbār, "the Dog of the Giant". Among the Merazig of Tunisia, shepherds note six constellations that mark the passage of the dry, hot season. One of them, called Merzem, includes the stars of Canis Major and Canis Minor and is the herald of two weeks of hot weather.

In Chinese astronomy, the modern constellation of Canis Major is located in the Vermilion Bird (南方朱雀; Nán Fāng Zhū Què), where the stars were classified in several separate asterisms of stars. The Military Market (軍市; Jūnshì) was a circular pattern of stars containing Nu³, Beta, Xi¹ and Xi², and some stars from Lepus. The Wild Cockerel (野雞; Yějī) was at the centre of the Military Market, although it is uncertain which stars depicted what. Schlegel reported that the stars Omicron and Pi Canis Majoris might have been them, while Beta or Nu² have also been proposed. Sirius was Tiānláng (天狼), the Celestial Wolf, denoting invasion and plunder. Southeast of the Wolf was the asterism Húshǐ (弧矢), the celestial Bow and Arrow, which was interpreted as containing Delta, Epsilon, Eta and Kappa Canis Majoris and Delta Velorum. Alternatively, the arrow was depicted by Omicron² and Eta and aiming at Sirius (the Wolf), while the bow comprised Kappa, Epsilon, Sigma, Delta and 164 Canis Majoris, and Pi and Omicron Puppis.

Both the Māori people and the people of the Tuamotus recognized the figure of Canis Major as a distinct entity, though it was sometimes absorbed into other constellations. Te Huinga-o-Rehua, also called Te Putahi-nui-o-Rehua and Te Kahui-Takurua, ("The Assembly of Rehua" or "The Assembly of Sirius") was a Māori constellation that included both Canis Minor and Canis Major, along with some surrounding stars. Related was Taumata-o-Rehua, also called Pukawanui, the Mirror of Rehua, formed from an undefined group of stars in Canis Major. They called Sirius Rehua and Takarua, corresponding to two of the names for the constellation, though Rehua was a name applied to other stars in various Māori groups and other Polynesian cosmologies. The Tuamotu people called Canis Major Muihanga-hetika-o-Takurua, "the abiding assemblage of Takarua".

The Tharumba people of the Shoalhaven River saw three stars of Canis Major as Wunbula (Bat) and his two wives Murrumbool (Mrs Brown Snake) and Moodtha (Mrs Black Snake); bored of following their husband around, the women try to bury him while he is hunting a wombat down its hole. He spears them and all three are placed in the sky as the constellation Munowra. To the Boorong people of Victoria, Sigma Canis Majoris was Unurgunite (which has become the official name of this star), and its flanking stars Delta and Epsilon were his two wives. The moon (Mityan, "native cat") sought to lure the further wife (Epsilon) away, but Unurgunite assaulted him and he has been wandering the sky ever since.