A telephone survey of 500 members of the American Independent Party found that nearly 75% did not realise they had enrolled in a political party that opposes abortion rights and same sex marriage.
Hollywood star Moore, who has contributed money to and campaigned for President Barack Obama, wrongly registered with the AIP, a representative said.
The representative told the Los Angeles Times: “Demi Moore is not, nor has ever been, a member of the American Independent Party.”
The newspaper, which conducted the survey, said voters were confused by the word “independent” in the party’s name.
In California, voters who do not want to register with any party tick a box on the registration form marked “no party preference”.
Patrick Schwarzenegger, son of former California Governor Arnold Schwarzenegger, registered for the AIP in 2013.
A family spokesman said the 22-year-old plans to change his registration.
According to the Secretary of State’s office, the party has around 472,000 members – 2.7% of the state-wide total.
The Los Angeles Times said that mistaken registration could stop people from casting votes in the 7 June presidential primary – which is expected to be the most competitive in the state in recent years.
Voters affiliated with the AIP would only be allowed to vote for candidates on the party’s ballot, the newspaper reported.
The deadline to register or change registration status for the primary is 23 May.
The AIP dates back to 1967 when George Wallace, a segregationist, launched his second campaign for the White House.
Mr Wallace, who had run as a Democrat in 1964, helped create the party and ran on its ticket.
Today, the AIP – which also advocates building a fence only the US border – only exists in California.
Markham Robinson, chairman of the AIP executive committee, said: “We’re not segregationist anymore. What we are now is a conservative, constitutionalist party.”
Among other celebrities registered with the party were Big Bang Theory star Kaley Cuoco and former professional boxer Sugar Ray Leonard.
Their representatives told the Los Angeles Times they would re-register before the primary.
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