The common dolphin (Delphinus delphis) is the most abundant cetacean in the world, with an estimated global population of about six million. It is currently the only member of the genus Delphinus, the type genus of the subfamily Delphininae. This places it as a close relative of the bottlenose dolphins, humpback dolphin, striped dolphin, spinner dolphin, Clymene dolphin, spotted dolphin, Fraser's dolphin, the tucuxi, and the Guiana dolphin.

The common dolphin was once divided into two different species, the short-beaked common dolphin and the long-beaked common dolphin. These are now generally regarded as ecotypes. Recent research shows that many long-beaked populations worldwide are not closely related to one another, often originating from short-beaked ancestors, and they do not consistently share the same derived traits. For this reason, these forms are no longer classified as separate species.

Despite its name, the common dolphin is not widely considered the "archetypal" dolphin; that role is more often associated with the bottlenose dolphin, largely because of its frequent appearances in aquaria and the media.^{[clarification needed]} Nevertheless, the common dolphin held cultural significance in antiquity and frequently appeared in Greek and Roman art, such as in murals created by the Minoan civilization.

Common dolphin are medium-sized dolphins; adults range between 1.9 and 2.5 m (6.2 and 8.2 ft) long, and can weigh between 80–235 kg (176–518 lb), although the range between 80–150 kg (180–330 lb) is more common. Males are generally longer and heavier. The dolphin's coloration is unusual, with a dark back and white belly (countershading), while on each side is an hourglass pattern colored light grey, yellow, or gold in front and dirty grey in back. They have long, thin rostra with up to 50–60 small, sharp, interlocking teeth on each side of each jaw.

Despite the historic practice of lumping the entire genus Delphinus into a single species, these widely distributed dolphins exhibit a wide variety of sizes, shapes and colors. Indeed, over the past few decades, over 20 distinct species in the genus have been proposed. Scientists in California in the 1960s concluded there were two species — the long-beaked and short-beaked. The long-beaked common dolphin was thought to have a disjointed range in coastal areas in tropical and warmer temperate oceans; this range included parts of western and southern Africa, much of western South America, central California to central Mexico, coastal Peru, the seas around Japan, Korea and Taiwan, and possibly near Oman. Vagrants have been recorded as far north as Vancouver Island.

This analysis was seemingly confirmed by a more in-depth genetic study in the 1990s. This study also suggested a third species (D. tropicalis, common name Arabian common dolphin or Indo-Pacific common dolphin), characterized by an extremely long and thin beak and found in the Red Sea and Indian Ocean, might be distinguished from the long-beaked species.

Recent evidence has demonstrated that different populations of long-beaked common dolphins around the world are not closely related to one another and are often descendant from a short-beaked ancestor (as well as not sharing common derived characters). Therefore, all long-beaked and short-beaked common dolphin populations are currently listed as the same species; they are all subspecies of Delphinus delphis. Currently, the common dolphin is divided into four subspecies:

Many extinct cetacean species were once lumped into Delphinus, but have since been moved to other genera. In the late 19th century several fossil species were described as part of this genus, such as Delphinus baltringii, Delphinus delannoy and Delphinus domeykoi. However, these species are no longer considered valid. Another species known as Delphinus brevidens was reassigned to the genus Stereodelphis, now generally considered synonymous with Squalodon.