Mahadeshwara Hill Saluru Math seer Pattada Immadi Mahadevaswamy was arrested and police told PTI said "he has a role" in the incident.
Additional Superintendent of Police of Chamarajnagar district, however, declined to divulge any other details.
Police sources said that at least five people, including the seer and a woman, have a direct role in the incident.
Officials have so far quizzed 11 people about the December 15 incident, in which more than 120 people fell ill after consuming prasad distributed at Maramma temple in Sulavadi village of Hanur taluk in Chamarajnagar district.
On the day of the incident, nine people died, while six others lost their lives over the next four days. 27 people are still in a critical state.
Lab test has found insecticide in the prasad. Police are suspecting a faction feud within the management of the temple over revenue sharing, which might have led to the poisoning.
(With inputs from agencies.)