X was an experimental submarine built for the French Navy (Marine nationale) during the first decade of the 20th century.

Designed by Gaston Romazzotti, she was shaped like the letter Y with two sterns, making her the first submarine with two propeller shafts.

The naval architect and constructor Gaston Romazzotti proposed in late 1900 to build a single-hull submarine with an experimental closed-cycle diesel engine that could propel the boat both on the surface in normal fashion and underwater using compressed air.

His proposal was accepted by the Board of Construction (Conseil des travaux) and approved by Navy Minister Jean Marie Antoine de Lanessan with the name X.

X was fitted with a telescoping tower that elevated the captain above the hull until the submarine was fully submerged.

[6] The submarine had two shafts, each powered by a Panhard & Levassor benzole engine for surface running with a designed combined total of 260 metric horsepower (191 kW) and two Sautter, Harlé electric motors which together produced 230 metric horsepower (170 kW) for submerged propulsion.

During the second transit in April 1906, both benzole engines broke down, one with a cracked cylinder and the other with a broken crankshaft; neither had been repaired as of 1907, which limited her to electric power only.