Wood drowned during a boat trip with her husband, TV star Robert Wagner, and actor Christopher Walken. At the time it was ruled to have been an accident.
But police reopened their inquiry last year after receiving new evidence.
The death certificate now says she died as a result of "drowning and other undetermined factors".
The amended document, obtained by the Associated Press news agency, also says the circumstances of how she ended up in the water are "not clearly established". The certificate was altered earlier this month.
Chief of detectives William McSweeney said the decision was made by the coroner's office. He added that detectives still had work to do on the case, but that did not necessarily mean a major development was coming.
"We don't close these cases," he said. "These cases have active periods and more passive periods. We're moving toward the end of an active period."
Conflicting versions of what happened on the yacht have contributed to the mystery of how the actress died in November 1981.
Wood had been partying the night before her death, and the coroner's investigation ruled she had been drinking and may have slipped trying to board the dinghy.
Resuming the investigation last November, the sheriff's office said: "Recently sheriff's homicide investigators were contacted by persons who stated they had additional information about the Natalie Wood Wagner drowning."
The move came hours after the captain of the boat, Dennis Davern, told NBC News that he lied to police during the initial investigation and that a fight between Wood and Wagner had led to her death.
In his book Pieces of My Heart, Wagner acknowledged that there had been a fight with Wood before she had disappeared, but authorities have said Wagner is not a suspect in his wife's death.
Wood's body was found floating in a Catalina Island cove off the coast of California. Police reports said she was found wearing a long nightgown, socks, and a jacket.
The post-mortem report said Wood had bruises on her body and arms as well as a facial abrasion on her left cheek.
Wagner's family said they supported the reopening of the inquiry and trusted the detectives would "evaluate whether any new information relating to the death of Natalie Wood Wagner is valid, and that it comes from a credible source or sources other than those simply trying to profit from the 30-year anniversary of her tragic death".
As a child, Wood featured in films like Miracle on 34th Street and The Ghost and Mrs Muir.
She was nominated for a best supporting actress Oscar for her role in Rebel Without a Cause, and for best actress for Splendor in the Grass and Love with the Proper Stranger.
Actress Natalie Wood's death certificate has been amended to reflect some of the lingering questions surrounding the star's death in 1981.