The third HMS Polyphemus was a Royal Navy torpedo ram, serving from 1881 until 1903. A shallow-draft, fast, low-profile vessel, she was designed to penetrate enemy harbours at speed and sink anchored ships. Designed by Nathaniel Barnaby primarily as a protected torpedo boat, the ram was provided very much as secondary armament.

A torpedo ram is a type of torpedo boat combining a ram with torpedo tubes. Incorporating design elements from the cruiser and the monitor, it was intended to provide small and inexpensive weapon systems for coastal defence and other littoral combat.