Hornby Lighthouse, also known as South Head Lower Light or South Head Signal Station, is a heritage-listed active lighthouse located on the tip of South Head, New South Wales, Australia, a headland to the north of the suburb Watsons Bay. It marks the southern entrance to Port Jackson and Sydney Harbour, as well as lighting the South Reef, a ledge of submerged rocks. It is the third oldest lighthouse in New South Wales. Designed by Mortimer Lewis and listed on the (now defunct) Register of the National Estate and on the New South Wales State Heritage Register since 2 April 1999, with the following statement of significance:

A dominant Sydney landmark which appears to have been in continuous use since the 1840s as a controlling point for shipping entering and leaving Port Jackson. The building complex, designed by the Colonial Architect Mortimer Lewis in the early 1840s, is an architectural important example of an early Victorian public work associated with port activities.