Agartha (variously spelled as Agharta, Aghartta, Agharti, among many other spellings) is a legendary kingdom that is said to be located on the inner surface of the Earth. Though the exact story varies, as there are many different versions, it is usually said to be located in Central Asia and led by a powerful figure sometimes called the King of the World, who secretly influences the surface. It is related to the belief in a hollow Earth and has been a popular subject in esotericism, occultism, and the New Age since the late 19th century.

The term and concept dates to the 1870s, first introduced by the French writer and colonial official Louis Jacolliot in his 1873 book Les fils de Dieu. Jacolliot claimed that he had been given access to ancient 15,000-year-old Indian manuscripts which told of the ancient city of Asgartha, its rise, and its fall. The original idea did not involve an underground kingdom, but was said to be India's destroyed former capital city, and is closer to Norse mythology than Indian mythology in content. Jacolliot's book was popular in France, and the concept of Agartha gained traction. The concept was afterwards expanded upon by a variety of occultist writers, including Alexandre Saint-Yves d'Alveydre. Saint-Yves wrote on it in his book Mission de l'Inde en Europe, which portrayed Agartha as still existing within the Earth, where one could travel through astral projection.

The idea was popularized by Ferdynand Ossendowski's 1922 book Beasts, Men and Gods, which was heavily influenced by Saint-Yves's version and became the standard version of Agartha's myth. Some interpretations involve Nordicism or Aryanism. A derived belief is that of the Grand Lodge of Agartha, a concept in Theosophy and related movements, where a group of ascended masters who secretly control the world are said to reside in Agartha. For unclear reasons, it is frequently associated or confused with the Buddhist mythical kingdom Shambhala, alternatively seen as a rival power, with either Agartha as the good to Shambhala's evil, or both as evil.