In national politics (or low politics), regionalisation is a process of dividing a political entity — typically a country — into smaller regions, and transferring power from the central government to the regions. Opposite process is called unitarisation.

In International Relations regionalisation is actually the set of processes that lead states to work together in an international system on a regional scale (e.g. ASEAN, EU etc). From the realist perspective of International Relations it is actually an outward expansion from the nation state.

Behind the process of regionalisation lies the concept of regionalism. This can be seen as the normative aspects, or values, that underly regionalisation e.g. the (contested) European identity. However, it is also a theoretical tool for analysis of international relations. For example the concept of security regime in regional security would not be possible without the analytic tool of 'regionalism'.

Regionalisation and unitarisation are often confused with, respectively, decentralisation and centralisation.

In literature, regionalism, or local-color fiction, was a perspective of literature that gained popularity in America after the Civil War. Local-color writers depicted nearly every region of the United States, leading realism to their stories by describing customs and manners and re-creating dialects. Because these authors usually set their stories in their regions as they remembered them from their own youth, however, they often blended realism with nostalgic sentiment. Many Americans found this mixture palatable, and local-colr stories filled the pages of the leading magazines until the end of the nineteenth century.