Semantic MediaWiki (SMW) is an extension to MediaWiki that allows for annotating semantic data within wiki pages, thus turning a wiki that incorporates the extension into a semantic wiki. Data that has been encoded can be used in semantic searches, used for aggregation of pages, displayed in formats like maps, calendars and graphs, and exported to the outside world via formats like RDF and CSV.

Semantic MediaWiki was initially created by Markus Krötzsch, Denny Vrandečić and Max Völkel, and was first released in 2005. Its development was initially funded by the EU-funded FP6 project SEKT (CORDIS site), and was later supported in part by Institute AIFB of the University of Karlsruhe (later renamed the Karlsruhe Institute of Technology). Currently SMW is maintained by an open-source-community on GitHub with Jeroen De Dauw as one of the lead maintainers.

Every semantic annotation within SMW is a "property" connecting the page on which it resides to some other piece of data, either another page or a data value of some type, using triples of the form "subject, predicate, object".

As an example, a page about Germany could have, encoded within it, the fact its capital city is Berlin. On the page "Germany", the syntax would be:

which is semantically equivalent to the statement "Germany" "Has capital" "Berlin". In this example the "Germany" page is the subject, "Has capital" is the predicate, and "Berlin" is the object that the semantic link is pointing to.

However, the much more common way of storing data within Semantic MediaWiki is via MediaWiki templates which themselves contain the necessary SMW markup.

For this example, the "Germany" page could contain a call to a template called "Country", that looked like this:

The "Country" template would handle storing whatever the value of the parameter "Capital" is, using the property "Has capital". The template would also handle the display of the data. Semantic MediaWiki developers have estimated that 99% of SMW data is stored in this way.