Adversarial information retrieval (adversarial IR) is a topic in information retrieval related to strategies for working with a data source where some portion of it has been manipulated maliciously. Tasks can include gathering, indexing, filtering, retrieving and ranking information from such a data source. Adversarial IR includes the study of methods to detect, isolate, and defeat such manipulation.

On the Web, the predominant form of such manipulation is search engine spamming (also known as spamdexing), which involves employing various techniques to disrupt the activity of web search engines, usually for financial gain. Examples of spamdexing are link-bombing, comment or referrer spam, spam blogs (splogs), malicious tagging. Reverse engineering of ranking algorithms, advertisement blocking, and web content filtering may also be considered forms of adversarial data manipulation.

Activities intended to poison the supply of useful data make search engines less useful for users. If search engines are more exclusionary they risk becoming more like directories and less dynamic.