HMS H6 was a British H-class submarine of the Royal Navy built by Canadian Vickers & Co. during World War I.

On 19 January 1916, she ran aground near the Dutch island of Schiermonnikoog, after which she was interned by the Royal Netherlands Navy.

The Royal Netherlands Navy renamed H6 to HNLMS O 8 and refitted her, with knowledge gained from the interned German submarine UC-8.

During the German attack on the Netherlands at the start of the Second World War, O 8 was still in Dutch service.

[11] After the surrender of the Netherlands, the German forces were able raise O 8 and found her almost fully intact.