It is a wild stream that originates from the Brahmagiri Hills which later joins River Kalindi. It is almost 400 mtrs away from the temple, on its western side. Literally it means, extinguisher of sins. It is believed that River Ganga and River Saraswathi join in Papanasini. Therefore Papanasini is called the Southern Kashi. A ritual dip in Papanasini is believed to wash one away, of all worldly sins committed in a life time. If we immerse the ashes of the dead in Papanasini, it is equivalent to that of doing Karmas(Rituals ) in Gaya.
At Papanasini there is a sacred rock called Pinnappara where ritual offerings to the spirits of the departed are made (known as bali). People believe that Parasurama, the famous incarnation of Lord Vishnu visited Thirunelli and performed last rites at the death of his father sage Jamadagni. He also took immersion in the Papanasini to wipe away sins committed owing to the annihilation of Kshatriyas. The ashes of Rajiv Gandhi was immersed in Papanasini in 1991.
Papanasini (destroyer of sins) has an interesting story to reveal. When Brahma was consecrating Vishnu’s idol Garuda (the carrier of Vishnu) reached over the sky, above the installation venue with the Amrit Kumbha (pot of nectar). During Garuda’s circling over, a drop of Amrit fell on the stream, which provided purifying power to the water in the stream. After that incident the stream came to be called Papanasini. Papanasini later joins in River Kalindi which is a tributary of River Kabani which later joins in River Kaveri.