Naturalisation (or naturalization) is the ecological phenomenon through which a species, taxon, or population of exotic (as opposed to native) origin integrates into a given ecosystem, becoming capable of reproducing and growing in it, and proceeds to disseminate spontaneously. In some instances, the presence of a species in a given ecosystem is so ancient that it cannot be presupposed whether it is native or introduced.

Generally, any introduced species may (in the wild) either go extinct or naturalise in its new environment.

Some populations do not sustain themselves reproductively, but exist because of continued influx from elsewhere. Such a non-sustaining population, or the individuals within it, are said to be adventive. Cultivated plants are a major source of adventive populations.

The above refers to naturalize as an intransitive verb, as in, "The species naturalized". In North America it is common to use naturalize as a transitive verb, as in, "City staff naturalized the park". This means to allow an environment to revert to its natural state.