The search giant argues that it has many gay, lesbian and bisexual employees, and that many come to work for the company because they can get married in California.
If these marriages are annulled, Google said that it would lose key talent and be placed at a competitive disadvantage.
"In the wake of the election many were concerned with the impact Proposition 8 could have on the personal lives of people they work with every day, and on California's ability to attract and retain a diverse mix of employees from around the world," said Google on its blog.
"That's why we've signed an amicus brief in support of several cases currently challenging Proposition 8 in the California Supreme Court.
"Denying employees basic rights isn't right, and it isn't good for businesses. We are committed to preserving fundamental rights for every one of the people who work hard to make Google a success."
Google also argues that, if Proposition 8 is allowed to stand, it will cause huge problems internally with payment systems. Health insurance, tax benefits and taxation systems based on the current recognition of gay marriage would have to be reorganised, at considerable cost.
Proposition 8 makes gay marriage illegal under the California constitution, and was passed by a narrow majority in November after a $20m (£13 m) campaign by churches, particularly the Mormons, to get it through.
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