Clashes were reported across the state after the two women activists, escorted by police, entered the Sabarimala temple in a surprise pre-dawn operation on Wednesday.
The Supreme Court in September overturned a decades-old ban on women of menstruating age - deemed as those between 10 and 50 - setting foot inside the gold-plated Sabarimala temple.
Two women defy centuries-old ban by entering Indian temple
Several women activists have made unsuccessful attempts to reach the temple since the order but faced stiff resistance from thousands of devotees including men and women, who see it as an attack on tradition.
"The person who died was part of a BJP (Bharatiya Janata Party) demonstration yesterday and got injured when some stones were hurled (at the demonstrators)," Kerala police spokesperson Pramod Kumar told AFP.
"His injuries were serious and he died late Wednesday night. At least 15 others were injured in incidents across the state," Kumar added.
Local media reports said the demonstrators from Prime Minister Narendra Modi's BJP were hit by stones from a local office of the state's ruling Communist party.
Kerala remained tense on Thursday, and the police said additional forces had been deployed across the state to prevent further violence breaking out.
The police Wednesday used tear gas, stun grenades and water cannon to control clashes between the rival groups, largely conservatives and cadres of the state's ruling left-wing parties.
Journalists were also assaulted during the disturbances in the state's capital, Thiruvananthapuram, and nearby Kollam city.
September's verdict was the latest progressive ruling from the court, with judges also overturning bans on gay sex and adultery last year.
In rare comments regarding the Sabarimala temple on Tuesday, Modi - running for a second term in elections later this year - appeared to support the ban, saying the matter was related to tradition.
Indian police fire tear gas at protesters after women enter temple
"There are some temples which have their own traditions, where men can't go. And men don't go," Modi told Indian media.
The Supreme Court is to start hearing a legal challenge on its ruling to allow women into the temple from January 22.
Women are still barred from a handful of Hindu temples in India. The entry of women at Sabarimala was taboo for generations and formalised by the Kerala High Court in 1991.
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