Pluto was discovered in 1930 by US astronomer Clyde Tombaugh
About 2,500 scientists meeting in Prague have adopted historic new guidelines that see the small, distant world demoted to a secondary category.
The researchers said Pluto failed to dominate its orbit around the Sun in the same way as the other planets.
The International Astronomical Union's (IAU) decision means textbooks will now have to describe a Solar System with just eight major planetary bodies.
There is a recognition that the demotion is likely to upset the public, who have become accustomed to a particular view of the Solar System.
"I have a slight tear in my eye today, yes; but at the end of the day we have to describe the Solar System as it really is, not as we would like it to be," said Professor Iwan Williams, chair of the IAU panel that has been working over recent months to define the term "planet".
Without a new nomenclature, these discoveries raised the prospect that textbooks could soon be talking about 50 or more planets in the Solar System.
Amid dramatic scenes in the Czech capital which saw astronomers waving yellow ballot papers in the air, the IAU voted to block this possibility - and in the process took the historic decision to relegate Pluto.
The scientists agreed that for a celestial body to qualify as a planet:
- it must be in orbit around the Sun
- it must be large enough that it takes on a nearly round shape
- it has cleared its orbit of other objects
Pluto was automatically disqualified because its highly elliptical orbit overlaps with that of Neptune. It will now join a new category of dwarf planets.
Pluto's status has been contested for many years. It is further away and considerably smaller than the eight other "traditional" planets in our Solar System. At just 2,360km (1,467 miles) across, Pluto is smaller even than some moons in the Solar System.
In addition, since the early 1990s, astronomers have found several objects of comparable size to Pluto in an outer region of the Solar System called the Kuiper Belt.
Some astronomers have long argued that Pluto would be better categorised alongside this population of small, icy worlds.
The critical blow for Pluto came with the discovery three years ago of an object currently designated 2003 UB313. After being measured with the Hubble Space Telescope, it was shown to be some 3,000km (1,864 miles) in diameter: it is bigger than Pluto.
2003 UB313 will now join Pluto in the dwarf category, along with Pluto's major moon, Charon, and the biggest asteroid in the Solar System, Ceres.
Named after the god of the underworld in Roman mythology, Pluto orbits the Sun at an average distance of 5.9 billion kilometres (3.7 billion miles) taking 247.9 Earth years to complete a single circuit of the Sun.
An unmanned US spacecraft, New Horizons, is due to fly by Pluto and the Kuiper Belt in 2015.