Spamdexing (also known as search engine spam, search engine poisoning, black-hat search engine optimization, search spam or web spam) is the deliberate manipulation of search engine indexes. It involves a number of methods, such as link building and repeating related or unrelated phrases, to manipulate the relevance or prominence of resources indexed in a manner inconsistent with the purpose of the indexing system.

Spamdexing could be considered to be a part of search engine optimization, although there are many SEO methods that improve the quality and appearance of the content of web sites and serve content useful to many users.

Search engines use a variety of algorithms to determine relevancy ranking. Some of these include determining whether the search term appears in the body text or URL of a web page. Many search engines check for instances of spamdexing and will remove suspect pages from their indexes. Also, search-engine operators can quickly block the results listing from entire websites that use Spamdexing, perhaps in response to user complaints of false matches. The rise of Spamdexing in the mid-1990s made the leading search engines of the time less useful. Using unethical methods to make websites rank higher in search engine results than they otherwise would is commonly referred to in the SEO (search engine optimization) industry as "black-hat" SEO. These methods are more focused on breaking the search-engine-promotion rules and guidelines. In addition to this, the perpetrators run the risk of their websites being severely penalized by the Google Panda and Google Penguin search-results ranking algorithms.

Common spamdexing techniques can be classified into two broad classes: content spam (term spam) and link spam.

The earliest known reference to the term spamdexing is by Eric Convey in his article "Porn sneaks way back on Web", The Boston Herald, May 22, 1996, where he said :

The problem arises when site operators load their Web pages with hundreds of extraneous terms so search engines will list them among legitimate addresses. The process is called "Spamdexing," a combination of spamming—the Internet term for sending users unsolicited information—and "indexing."

Keyword stuffing had been used in the past to obtain top search engine rankings and visibility for particular phrases. This method is outdated and adds no value to rankings today. In particular, Google no longer gives good rankings to pages employing this technique.

Hiding text from the visitor is done in many different ways. Text colored to blend with the background, CSS z-index positioning to place text underneath an image — and therefore out of view of the visitor — and CSS absolute positioning to have the text positioned far from the page center are all common techniques. By 2005, many invisible text techniques were easily detected by major search engines.^{[citation needed]}