The Church of St John sub Castro is an Anglican church in Lewes, the county town of East Sussex, England (grid reference TQ 414 104). It was built in 1839 on the site of an 11th-century Saxon church, and has been designated by English Heritage as a Grade II listed building. In the churchyard is a memorial to Finnish prisoners from the Crimean War who died while confined in Lewes Naval Prison; the memorial is also listed Grade II.