Under current California law, death can be determined by the loss of all brain function and in accordance with accepted medical standards.
While Heche is legally dead, she remains on life support and "her heart is still beating" so the non-profit organisation OneLegacy can determine if she can be a donor, spokeswoman Holly Baird said in a statement.
The process, which involves assessing which organs are viable and finding an appropriate recipient, could take several days, Baird told The Associated Press.
In the US, most organ transplants are done after the donor has been declared brain-dead.
The actor, 53, suffered a "severe anoxic brain injury", caused by a lack of oxygen, when her car crashed into a Los Angeles area home on August 5 and fire erupted, according to a statement released on Thursday that said she was not expected to survive.
She has been hospitalised at a Los Angeles burns centre.
"This is a sad day. I'm sending Anne's children, family and friends all of my love," TV presenter Ellen DeGeneres tweeted on Friday amid reports of Heche's dire condition.
The pair were a couple from 1997 to 2000.
Detectives investigating the crash said narcotics were found in a blood sample taken from Heche, though toxicology tests still had to be performed to differentiate them from drugs she was given for her injuries, Los Angeles police said.
A native of Ohio, Heche came to prominence on the NBC soap opera Another World from 1987 to 1991. She won a Daytime Emmy Award for the role of twins Marley and Vicky.
In the late 1990s she became one of the hottest actors in Hollywood, a constant on magazine covers and in big-budget films.
In 1997 alone she played opposite Johnny Depp as his wife in Donnie Brasco, and Tommy Lee Jones in Volcano.
She was also part of the ensemble cast in the original I Know What You Did Last Summer.
The following year, she starred with Harrison Ford in Six Days, Seven Nights and appeared with Vince Vaughn and Joaquin Phoenix in Return to Paradise.
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