Google said in a court filing that its users "cannot be surprised" the company regularly accesses their e-mails.
Consumer Watchdog, the organization that unearthed the documents, called this a "stunning admission."
"Google has finally admitted they don't respect privacy," Consumer Watchdog spokesman John M. Simpson said in a statement. "People should take them at their word; if you care about your e-mail correspondents' privacy, don't use Gmail."
The filings stem from a class-action lawsuit launched against the company in May over Gmail's practice of routinely scanning e-mails for keywords to target advertising at users.
The lawsuit claims Google "unlawfully opens up, reads and acquires the content of people's private e-mail messages."
But Google claims the practice, which is laid out in its terms of service, passes the "reasonable expectation of privacy" test established by the U.S. Supreme Court in 1967. In that case, the court ruled that wiretaps are unconstitutional because there is a "reasonable expectation" that phone calls are private.
Not so with e-mails, Google says.
"Just as a sender of a letter to a business colleague cannot be surprised that the recipient's assistant opens the letter, people who use Web-based e-mail today cannot be surprised if their e-mails are processed by the recipient's (e-mail provider) in the course of delivery."
Quoting a 1979 Supreme Court case that allowed phone companies to install devices that track which phone numbers have been dialed, Google said: "A person has no legitimate expectation of privacy in information he voluntarily turns over to third parties."