Nathamuni (823 CE–951 CE) was a Vaishnava theologian who collected and compiled the Nalayira Divya Prabandham. Considered the first of Sri Vaishnava āchāryās, Nathamuni is also the author of Yogarahasya, and Nyāyatattva.
Nathamuni is generally considered to have been born in 823 AD and to have died in 951 AD. His birth name was Aranganathan however he was known as Nathamuni or literally the Saint lord (Nathan-lord, muni-saint) An alternative view is that he was born in 582 AD and died in 922 AD. Yet another view is that Nathamuni was born at Viranarayana Puram sometime shortly after 907 AD and flourished in the 10th century. The traditional view that he lived for than 400 years is untenable. It is likely that Nathamuni lived for slightly over a hundred years in that region controlled by the Chola kings before they rose to the peak of their greatness. His birth star was Anusham.
Though there is difficulty in fixing Nathamuni's date of birth and age, he is considered to have lived during the lifetime of Madhurakavi Alvar's Parampara. That Nathamuni was in contact with Nammalvar is attested by the Guru-paramparā, Divya sūri charita, and Prappannāmŗta. The Prappannāmŗta also attests that Nathamuni was born in the village Viranarayana. Viranarayana is today generally identified as Kattumannarkoil. Nathamuni is said to have died at Gangaikonda Cholapuram. His father's name was Iśvara Bhaṭṭa and his son's name was Iśvaramuni. His grandson was Yamunacharya  who was probably named in commemoration of a pilgrimage that Nathamuni took to the banks of the Yamuna along with his son (Iśvara Muni) and daughter-in-law.
It is believed that his other names were Sadamarsana Kula Tilakar, Sottai Kulaththu Arasar and Ranganatha Acharya.
He spent time travelling in north India. He came to know about Nalayira Divya Prabhandam, but he heard only 10 hymns. He wanted the rest. He recited 12000 times, Kanninun Siruthambu, a poem in praise of Nammazhwar. Nammazhwar appeared and gave the 4000 hymns(Nalayira Divya Prabhandam). He was the one who brought back the 4000 hymns. In addition to teaching the hymns to his two nephews at Srirangam, he introduced them into the Srirangam Temple Service at the Sri Ranganathaswamy Temple, Srirangam where he was the Temple Administrator.
The ritual of worship as followed in Vishnu Temples is based on two early standard works. The first being Vaikhanasa Sutra which probably belongs to the Krishna Yajurveda school. The other is the Panchratra Agama which belongs to the extensive Tantra literature, believed to have been composed by Narayana himself. The Agama has a peculiar philosophy of its own, the Bhagavata Cult, which is very old and is referred to in the Mahabharata and Badarayana Sutras. Nathamuni, made a provision for the recitation of Tamil vedas on appropriate occasions during the main festivals of the lord.
Nammazhwar's songs are sung to this day at Srirangam and other places of where Vishnu is worshipped. Nathamuni is said to have set them into music after his discovery of these verses. During that period, a dancing girl sang songs in the same celestial tune (in which Nathamuni set the prabandhams into music) at the court of the Chola king in Gangaikondacholapuram. The tune was rare and could not be appreciated by the common folk and hence the king slighted the dancer. The dancer travelled to the Veeranarayanapuram vishnu temple and sang before God in the same celestial tune. This was appreciated by Nathamuni who understood the nuances of the tune. On hearing that Nathamuni himself had appreciated the dancer's singing, the king visited the temple and enquired why Nathamuni had appreciated that unfamiliar tune. To display his prowess, Nathamuni ordered several cymbals to be sounded and determined the weight of the cymbals from the pitch of the sound that they produced. This impressed the king and he accepted the superiority of the celestial tune.
There is an inconsistency in this anecdote. During the time of Nathamuni (late 9th century), Uraiyur was the capital of the Chola kings, and Gangaikondacholapuram had not been founded yet. However, it is possible that the site of the city was used as an alternative capital or had a palace that was frequented by the kings.
One of Nathamuni's most illustrious disciples was Pundarikaksha, who hasn't left any literary work behind him. It is believed that Nathamuni had a vision where he foresaw the birth of his grandson Yamunacharya and deputed Pundarikaksha to be his spiritual guru (who in turn deputed his disciple Ramamisra to guide Yamunacharya).
It is said that Nathamuni once asked Pundarikaksha to escort his wife Aravindappavai to the residence of her father Vangi-purathachi. On reaching the house of Vangi-purathachi he was served stale food as he was from an inferior caste among Brahmins (Choliah). Yet he never resented the apparent slight and indignity, but accepted it cheerfully. When Nathamuni heard of this incident, he realised that it was a mark of high spiritual advancement and called him by the name of Uyyakondar - “Savior of the new dispensation”.