Phobos, Greek for "fear", is the root word of phobia. It may refer to:

• Phobos (mythology), one of the sons of Ares and Aphrodite in Greek mythology
• Phobos (moon), the larger and innermost of Mars' two moons
• Phobos (physics) is a nuclear physics experiment
• Phobos program, consisting of two space probes, Phobos 1 and Phobos 2, sent to Mars by the Soviet Union
• Phobos (album), the ninth album by Canadian metal/cyberpunk band Voivod
• Project Phobos is a lightweight web application environment running on the Java platform
• Phobos, the standard runtime library of the D programming language

In fiction:

• Phobos (Doctor Who audio) is an audio drama based on the television series Doctor Who
• Phobos (W.I.T.C.H.), a villain in W.I.T.C.H., the unrightful ruler of Meridian in the first saga
• Phobos (known as Huitzil outside Japan), a character in Capcom's fighting game series Darkstalkers
• Phobos, a Marvel Comics character and member of the Secret Warriors
• Phobos, one of three primary locations in the video game Doom