The site was first blocked on Tuesday, the broadsheet said, although Chinese netizens were still able to access its mobile and iPad apps.
"Obviously we are dismayed that theguardian.com has been blocked in China," a spokeswoman for the newspaper said.
"The reason for this is currently unclear but we are investigating the extent of the block and hope that access to our website will return to normal in the very near future."
The Guardian, which has one of the world's most read newspaper websites, insisted it had not provoked the block by publishing any controversial stories about China in recent days.
China's ruling Communist Party -- highly sensitive to social unrest that might challenge its authority -- tightly controls the Internet and news media.
Several Western news organisations have accused China of blocking access to their websites in the past, including the New York Times, Wall Street Journal, Bloomberg and Reuters.
The Guardian said on its website: "No China-related stories published by the Guardian in the past two days would obviously be perceived as dangerous by the country's leadership.
"One article, published on 6 January, explores tensions in China's ethnically-divided north-western region Xinjiang, but the Guardian has covered the subject before without any noticeable fallout."
The newspaper said attempts to access the site from multiple browsers, devices and locations across Beijing had failed without the use of firewall-dodging software.
China employs a so-called Great Firewall to filter content from abroad, and a huge staff to rapidly delete offending articles and microblog posts. The popular sites Facebook, Twitter and YouTube are inaccessible.Chinese censors routinely delete online content deemed sensitive, but it blocks access to entire websites less frequently.