The attack knocked WikiLeaks.org offline for more than three hours Tuesday night and early Wednesday London time, the Financial Times reported.
Visitors told the Times the site was inaccessible from parts of Europe, North America and Australia.
Several back up sites were created so WikiLeaks could continue distribution of the confidential material.
The organization's latest document dump has been condemned by authorities in the United States and Australia for appearing to reveal the names of terror suspects and government informants.
"For the first time, the diplomatic cables are available from every country that has U.S. diplomatic representation," WikiLeaks said before releasing the documents.
Robert McClelland, Australia's attorney general, criticized WikiLeaks for failing to redact identifying features that might put security operations or safety at risk.