Sui generis is a (post) Latin expression, literally meaning of its own gender/genus or unique in its characteristics. The expression was effectively created by scholastic philosophy to indicate an idea, an entity or a reality that cannot be included in a wider concept, and in the structure genus > species a species that heads its own genus.

In law, it is a term of art used to identify a legal classification that exists independently of other categorizations because of its uniqueness or due to the specific creation of an entitlement or obligation. In intellectual property there are rights which are known as being sui generis to owners of a small class of works, such as intellectual property rights in mask works, ship hull designs, databases, or plant species.

In British town planning law, certain uses of land are labelled "sui generis" to indicate that they are outside a legally-defined Use Class, effectively in a class of their own. Such uses include laundrettes, taxi or vehicle hire businesses, amusement centres, petrol filling stations, hostels and theatres. Under this style of regulation, changes of land use within an agreed "Use Class" can be undertaken without an explicit application being made to a local municipal authority. Changes in the use of a property or site across the boundaries of those Use Classes, or to a "sui generis" activity, can thus be subject to some democratic influence.

The term has also been used in the context of aboriginal law in Canada to describe the nature of aboriginal title .