"I'm fasting tomorrow to support the spirit of Tamil Nadu," Rahman posted on Twitter on Thursday.
Various artistes of the Tamil film fraternity will participate in the fast in support of Jallikattu, the ancient and popular bull-taming sport.
Thousands of young men and women here have demanded not only an end to the ban on Jallikattu but also a ban on People for Ethical Treatment of Animals (PETA), which opposes the sport.
In Jallikattu, a bull vaulter is expected to hang on to the animal's hump for a stipulated distance or for a minimum of three jumps by the bull.
The Supreme Court banned Jallikattu in May 2014, saying that bulls cannot be used as performing animals including bullock-cart races. Since then, people have been urging the central government to take steps to allow the sport.
Various celebrities from southern cinema, including Rajinikanth, Kamal Hassan and Vetrimaaran have supported the protest against the ban.