In SPARQL, a valid URI is a Uniform Resource Identifier that uniquely identifies a resource, such as a web page, a file, or a concept. URIs in SPARQL are used to represent entities in a dataset, such as subjects, predicates, and objects in triples. URIs must follow the syntax rules for URIs, which typically include a scheme (such as "http://" or "urn:"), a host name, and a path. Additionally, URIs in SPARQL can be mapped to namespaces for easier referencing in queries.