The 108 Upanishads are a collection of ancient Sanskrit texts that contain some of the central philosophical concepts of Hinduism, such as the idea of Atman (the self or soul) and Brahman (the absolute reality). They are also known as the Vedanta (the end or conclusion of the Vedas), which is one of the six systems of Indian philosophy. The Upanishads document the transition from the ritualistic Vedic religion to a more mystical and meditative vision of spirituality. ¹²

The Upanishads are divided into two categories: the principal or major Upanishads, which are the oldest and most important, and the minor or new Upanishads, which are later and less authoritative. The principal Upanishads are 12 to 14 in number, depending on the tradition, and include texts such as the Brihadaranyaka, the Chandogya, the Katha, the Mundaka, and the Taittiriya. The minor Upanishads are more than 90 in number, and include texts such as the Yoga, the Narada Parivrajaka, and the Surya. ¹³

The Upanishads explore various themes such as the nature of reality, the origin and purpose of life, the relation between the individual and the universal, ethical, and spiritual values, and the methods of attaining liberation from suffering. The Upanishads are not systematic treatises, but rather dialogues, stories, poems, and hymns that express different perspectives and insights. The Upanishads have influenced many schools of Hindu thought, such as Advaita (non-dualism), Vishishtadvaita (qualified non-dualism), and Dvaita (dualism), as well as other religions such as Buddhism, Jainism, Sikhism, and even Western philosophy. ¹²

The 108 Upanishads are given in the Muktika Upanishad, which is a late text that claims to be the last Upanishad. The list of 108 Upanishads is divided into five categories based on the Vedas they belong to and the topics they deal with. Here is the list of 108 Upanishads according to the Muktika Upanishad ¹²:

Rig Veda (10 Upanishads)

  1. Aitareya
  2. Kaushitaki
  3. Nadabindu
  4. Atmabodha
  5. Nirvana
  6. Mudgala
  7. Akshamala
  8. Tripura
  9. Saubhagyalakshmi
  10. Bahvrichi

Sama Veda (16 Upanishads)

  1. Kena
  2. Chandogya
  3. Arunika
  4. Maitrayani
  5. Maitreyi
  6. Vajrasuchi
  7. Yogachudamani
  8. Vasudeva
  9. Mahat
  10. Sanyasa
  11. Avyakta
  12. Kundika
  13. Savitri
  14. Rudrakshajabala
  15. Jabaladarsana
  16. Jabali

Shukla Yajur Veda (19 Upanishads)

  1. Isavasya
  2. Brihadaranyaka
  3. Jabala
  4. Hamsa
  5. Paramahamsa
  6. Subala
  7. Mantrika
  8. Niralamba
  9. Trisikhibrahmana
  10. Mandalabrahmana
  11. Advayataraka
  12. Paingala
  13. Bhikshuka
  14. Turiyatita
  15. Adhyatma
  16. Yajnavalkya
  17. Satyayani
  18. Tarasara
  19. Muktika

Krishna Yajur Veda (32 Upanishads)

  1. Katha
  2. Taittiriya
  3. Brahma
  4. Kaivalya
  5. Svetasvatara
  6. Garbha
  7. Narayana
  8. Amritabindu
  9. Amritanada
  10. Kalagnirudra
  11. Kshurika
  12. Sarvasara
  13. Sukharahasya
  14. Tejobindu
  15. Dhyanabindu
  16. Brahmavidya
  17. Yogatattva
  18. Dakshinamurti
  19. Skanda
  20. Sariraka
  21. Yogasikha
  22. Ekakshara
  23. Akshi
  24. Avadhuta
  25. Katharudra
  26. Rudrahrudaya
  27. Yogakundalini
  28. Panchabrahma
  29. Pranaagnihotra
  30. Varaha
  31. Kalisantaranaand
  32. Sarasvatirahasya

Atharva Veda (31 Upanishads)

78. Prasna
79. Mundaka
80. Mandukya
81. Atharvasira
82. Atharvasikha
83. Brahajjabala
84. Nrsimhatapini
85. Narada Parivrajaka
86. Sita
87. Sarabha
88. Tribadvibhutimahanarayanaand
89. Ramarahasya
90. Ramatapini
91. Sandilya
92. Paramahamsaparivrajaka
93. Annapurna
94. Surya
95. Atma
96. Pasupatabrahma
97. Parabrahma
98. Tripuratapini
99. Devi
100. Bhavana
101. Bhasmajabala
102. Ganapati
103. Mahavakhya
104. Gopalatapini
105. Krishna
106. Hayagriva
107. Dattatreya
108. Garuda

References:
(1) The Names of 108 Upanishads, complete list – Hinduism Facts. https://www.hinduismfacts.org/the-names-of-108-upanishads/
(2) Upanishads – Wikipedia. https://en.wikipedia.org/wiki/Upanishads
(3) List of 108 Upanishads According To The Muktikopanishad – Hindu Website. https://www.hinduwebsite.com/upalist.asp
(4) Vedanta | Hindu philosophy | Britannica. https://www.britannica.com/topic/Vedanta
(5) Hinduism – The Upanishads | Britannica. https://www.britannica.com/topic/Hinduism/The-Upanishads