Under Wednesday's ruling by the 9th U.S. Circuit Court of Appeals, employers that contract an outside business to transmit text messages can't read them unless the worker agrees.
Users of text-messaging services "have a reasonable expectation of privacy" regarding messages stored on the service provider's network, Judge Kim Wardlaw wrote in the three-judge panel's unanimous opinion.
The ruling limits employers' access to employee e-mail on internal servers.
The text-message part of the ruling will affect more employers than the e-mail portion because most U.S. companies pay outside parties for text-messaging but keep e-mail on internal servers, analysts said.
The judges had few precedents, Wardlaw acknowledged in the ruling.
"The extent to which the Fourth Amendment provides protection for the contents of electronic communications in the Internet Age is an open question," she wrote.
A civil liberties advocacy group called the ruling a "tremendous victory" for online privacy. The Electronic Frontier Foundation said in a posting online that the ruling helps ensure the Fourth Amendment "applies to your communications online just as strongly as it does to packages and letters."
The ruling came in a lawsuit filed by Ontario police Sgt. Jeff Quon and three other officers after Arch Wireless gave their department transcripts of Quon's text messages in 2002. Police officials read the messages to determine whether department-issued pagers were being used solely for work purposes.
"I think right now service providers are going to be a little leery of providing anything to the subscriber because of this case," said John Horowitz, a lawyer representing Arch Wireless.
Dimitrios Rinos, an attorney for the city of Ontario and its police department, said his clients probably will appeal the ruling.