In a full page statement published in The Sydney Morning Herald, Rugby Australia (RA) said the proposed referendum is "too important a contest to watch in silence".
"Provision for an Indigenous Voice to Parliament in our Constitution is long overdue," it added.
RA and the AFL join the National Rugby league (NRL), several AFL clubs, and other sports governing bodies like soccer's Football Australia, Tennis Australia and the Australian Olympic Committee in formalising support for Indigenous recognition through a referendum.
Prime Minister Anthony Albanese, who has staked significant political capital on the referendum, has been keen to get the support of leading sports bodies and players.
Some of Australia's top athletes are Indigenous, like NBA star Patty Mills, former Olympic sprinter Cathy Freeman and former woman's tennis world number one Ash Barty.