The extension, called Password Alert, can be downloaded on Google Chrome and warns users before they enter account information on "phishing" pages, or imitation sites designed to steal passwords and access personal information, such as emails or online bank accounts.
Millions of phishing emails and websites are sent every day, Google said. Nearly 2 percent of messages sent through Gmail, Google's email service, are designed to steal passwords.
"Right now, it's left up to the user to decide whether or not to enter their password," said Drew Hintz, the lead engineer for Password Alert. "We expect users to know the difference between these sites, and that's an unreasonable request to make of users."
The new extension, which took about three years to create, is an addition to Google's other security measures, including safe browsing technology that warns users against potentially malicious sites and verification tools that help protect private accounts.