HMS J6 was a First World War J-class submarine built for the Royal Navy by HM Dockyard at Devonport in Plymouth. Commissioned in 1916, she was sunk in a friendly fire incident by the Q-ship Cymric in October 1918.

The J-class submarines were seven submarines developed by the Royal Navy prior to the First World War in response to claims that Germany was developing submarines that were fast enough to operate alongside surface fleets. Six were completed during mid-1916, while a seventh entered service at the end of 1917.