HMS C7 was one of 38 C-class submarines built for the Royal Navy in the first decade of the 20th century. The boat survived the First World War and was sold for scrap in 1920.
The C class was essentially a repeat of the preceding B class, albeit with better performance underwater. The submarine had a length of overall, a beam of and a mean draft of . They displaced on the surface and submerged. The C-class submarines had a crew of two officers and fourteen ratings.
For surface running, the boats were powered by a single 16-cylinder Vickers petrol engine that drove one propeller shaft. When submerged the propeller was driven by a electric motor. They could reach on the surface and underwater. On the surface, the C class had a range of at .
The boats were armed with two 18-inch (45 cm) torpedo tubes in the bow. They could carry a pair of reload torpedoes, but generally did not as they would have to remove an equal weight of fuel in compensation.
C7 was built by Vickers at their Barrow-in-Furness shipyard, laid down on 9 December 1905 and was commissioned on 23 May 1907. The boat is often mistakenly credited with sinking on 5 April 1917 off the coast of The Netherlands. However, this attack was made against , which survived. C7 was sold on 20 December 1919.