Kartika Purnima (Sanskrit: कार्त्तिकपूर्णिमा, romanized: Kārttikapūrṇimā), also known as Kartika Pournami, is a Hindu, Sikh, and Jain cultural festival that is celebrated on purnima (full moon day), the 15th day of the lunar month Kartika. It falls in November or December of the Gregorian calendar and is also known as Tripurari Purnima or Deva-Deepavali, the gods festival of lights. Karthika Deepam is a related festival that is celebrated in South India and Sri Lanka on a different date. It follows Diwali by about 15 days.

In Vaishnavite tradition, this day is considered significant and special for the worship of both Radha and Krishna. It is believed that on this day, Radha-Krishna performed rasalila with their gopis. At Jagannath Temple, Puri and all other Radha-Krishna temples, a sacred vow is observed throughout Kartika month, and performances of raaslila are organized on the day of Kartika Purnima. According to Brahma Vaivarta Purana, Krishna worshipped Radha on this day.

'Tripuri Purnima' or 'Tripurari Purnima' derives its name from Tripurari – the foe of the demon Tripurasura. In some legends of Kartika Purnima, the term is used to denote the three demon sons of Tārakāsura. Tripurari is an epithet of the god Shiva. Shiva, in his form as Tripurantaka (lit. 'Killer of Tripurasura'), killed Tripurasura on this day. Tripurasura had conquered the whole world and defeated the gods and also created three cities in space, together called "Tripura". The killing of the demon(s) and destruction of his/their cities with a single arrow by Shiva overjoyed the gods, and they declared the day as a festival of illuminations. This day is also called "Deva-Diwali"—the Diwali of the gods.

Kartika Purnima is also celebrated as the manifestation day of Matsya, the god Vishnu's fish incarnation (avatar) and Vrinda, the personification of the tulasi. In the Brahma Vaivarta Purana, Tulasi is a gopi cursed to be born on earth as a princess and marry Shankacuda, an asura. She remained a devotee of Vishnu and strongly wished to marry the deity. When Shankacuda threatens the existing order with his conquests, invincible owing to the chastity of his wife, Vishnu assumes his form and tricks her into hugging him, causing her to lose her chastity and allowing Shiva to triumph in his battle over the asura. Vishnu then revealed his true form and married her. Tulasi cursed that Vishnu would take the form of a shaligrama stone for his trickery, and to appease her, the deity stated that her essence would become the tulasi plant and the Gandaki river. Her divine form ascended to Vaikuntha to be with him.

In Southern India, Kartika Purnima is also celebrated as the birthday of Kartikeya, the god of war and younger son of Shiva. This day is also dedicated to the pitrs, dead ancestors.

Underhill believes that the origins of this festival may lie in ancient times, when a sacrifice called Shakamedhah was performed to attain victory over enemies.

The festival has even more significance when the day falls in the nakshatra (lunar mansion) of Krittika and is then called Maha Kartika. If the nakshatra is Bharani, the results are stated to be special. If it is Rohini, then the fruitful results are even more. Any philanthropic act on this day is supposed to bring benefits and blessings equal to the performing of ten yajnas.

Kartika Purnima is closely associated with Prabodhini Ekadashi, which marks the end of the chaturmasya, a four-month period when Bhagwan Vishnu is believed to sleep. Prabodhini Ekadashi signifies the awakening of the god. Many fairs that begin on Prabodhini Ekadashi end on Kartika Purnima, Kartika Purnima usually being the most important day of the fair. Fairs that conclude on this day include Prabodhini Ekadashi celebrations at Pandharpur and Pushkar Fair. Kartika Purnima is also the last day to perform the Tulasi Vivaha ceremony, which can be performed from Prabodhini Ekadashi.