Researchers from the firm surveyed billions of sites, subjecting 4.5 million pages to "in-depth analysis".
About 450,000 were capable of launching so-called "drive-by downloads", sites that install malicious code, such as spyware, without a user's knowledge.
A further 700,000 pages were thought to contain code that could compromise a user's computer, the team report.
To address the problem, the researchers say the company has "started an effort to identify all web pages on the internet that could be malicious".
Drive-by downloads are an increasingly common way to infect a computer or steal sensitive information.
They usually consist of malicious programs that automatically install when a potential victim visits a booby-trapped website.
"To entice users to install malware, adversaries employ social engineering," wrote Google researcher Niels Provos and his colleagues in a paper titled The Ghost In The Browser.
As well as characterising the scale of the problem on the net, the Google study analysed the main methods by which criminals inject malicious code on to innocent web pages.
It found that the code was often contained in those parts of the website not designed or controlled by the website owner, such as banner adverts and widgets.
Widgets are small programs that may, for example, display a calendar on a webpage or a web traffic counter. These are often downloaded from third-party sites.
The rise of web 2.0 and user-generated content gave criminals other channels, or vectors, of attack, it found.
For example, postings in blogs and forums that contain links to images or other content could unwittingly infect a user.
The study also found that gangs were able to hijack web servers, effectively taking over and infecting all of the web pages hosted on the computer.
In a test, the researchers' computer was infected with 50 different pieces of malware by visiting a web page hosted on a hijacked server. The firm is now in the process of mapping the malware threat.
Google, part of the StopBadware coalition, already warns users if they are about to visit a potentially harmful website, displaying a message that reads "this site may harm your computer" next to the search results
