Fifth planet may refer to:

Astronomy

• Jupiter, fifth planet from the Sun in our Solar System
• Fifth planet (hypothetical), any of various hypothetical planets thought to have existed
• Planet V, 2002 scientific proposal for a destroyed fifth planet
• Mars, fifth planet from Earth in the Ptolemaic geocentric model
• Ceres (dwarf planet), dwarf planet between Mars and Jupiter in the asteroid belt

Fiction

• Fifth Planet (novel), science fiction novel by Fred and Geoffrey Hoyle