The Cathedral Church of St John the Evangelist (also known as St John's Cathedral) is a Roman Catholic cathedral in Portsmouth, England.

When the Diocese of Portsmouth was formed in May 1882, St. John's was named the cathedral. The cathedral was consecrated by the first bishop of the new diocese, John Vertue, on 29 March 1887. He enlarged the cathedral and completed its interior decorations.

The cathedral was designed in Gothic Revival style by John Crawley in 1877–1881. Crawley died just as building started, and his partner Joseph Hansom took over the project and modified the design, working on it until 1896.