The high court filed an order Thursday approving the changes. It provided no explanation for its decision.
California currently bars attorneys from coercing a client into sex or demanding sex in exchange for legal representation.
The new rule would completely ban sex between lawyers and clients unless the sexual relationship preceded the professional relationship or the client is the lawyer's spouse or domestic partner.
The proposal by California's State Bar was divisive, even though at least 17 other states have adopted a similar ban.
Another rule approved by the state Supreme Court allows the state bar to discipline attorneys for discrimination and harassment even without a separate finding of wrongdoing.