犬（学名：Canis lupus familiaris），现代俗称为狗，一种常见的犬科哺乳动物，生物学分类上是狼的一个亚种。狗是最早被人类驯养的一个物种。被人豢养的称为家犬，返回野外没人养的狗称为“野狗”或“流浪狗”。犬的寿命约十多年，与猫的平均寿命相近。若无发生意外，平均寿命以小型犬为长。

DNA分析显示狗不单单是灰狼（Canis lupus）的近亲，更是各地不同灰狼亚种的混血后代。现代狗的祖先约在1.1万至1.6万年前开始在西欧被人类驯化，更早的古代狗如在比利时所发现距今3.6万年及阿尔泰山所发现约3.3万年前的标本，则显示已经中断的驯化事件，这些更早期的古代狗均未留下现存后代。

The domestic dog (Canis lupus familiaris or Canis familiaris) is a domesticated canid which has been selectively bred for millennia for various behaviors, sensory capabilities, and physical attributes.

Although initially thought to have originated as an artificial variant of an extant canid species (variously supposed as being the dhole, golden jackal, or gray wolf), extensive genetic studies undertaken during the 2010s indicate that dogs diverged from an extinct wolf-like canid in Eurasia 40,000 years ago. Being the oldest domesticated animal, their long association with people has allowed dogs to be uniquely attuned to human behavior, as well as thrive on a starch-rich diet which would be inadequate for other canid species.