Monturiol had already named his vessel Ictineo, from the ancient Greek icthus (fish) and naus (boat). As he put it, the Ictíneo’s "form is that of a fish, and like a fish it has its motor in the tail, fins to control its direction, and swimming bladders and ballast to maintain an equilibrium with the water from the moment it submerges".
Monturiol realized that the ideal shape for such a vessel from the point of view of hydrodynamics and steerage was that of a fish. However, the optimum shape of hull to withstand water pressure was a sphere. He therefore combined the two, with an inner, ellipsoidal pressure hull and an outer, fish-shaped hull which was open to the sea and free-flooding. Ballast tanks and any other equipment which interacted with the sea were to be placed in the space between the two hulls.
In September 1857 he returned to Barcelona, where he organized the first commercial society in Spain dedicated to submarine navigation, Monturiol, Font, Altadill y Cia. with a capital of 10,000 pesetas. In 1858 he presented his project in a scientific thesis titled The Ictineo or fish-ship.
On 28 June 1859, Monturiol was ready for the Ictineo's first voyage and the submarine was launched into Barcelona harbour. Unfortunately, she hit some underwater pilings, which Monturiol estimated would exhaust his funds to properly repair. He performed some hasty repairs on the damaged portholes, exterior hull, and ballast tanks, and limited his diving depth to .
During the summer of 1859 Monturiol performed more than 20 test dives in the Ictineo, with his business partner and shipbuilder as crew. He gradually increased the depth he dived to until he reached his limit and learned that the crew could remain dived for about 2 hours using only the oxygen sealed inside the pressure hull, and that their endurance could be doubled using compressed oxygen and his carbon dioxide scrubber. The Ictineo turned out to possess good handling, but its top speed was disappointing, powered as it was by human muscle power.
Ictineo was eventually destroyed in January 1862 after some 50 dives, when a cargo vessel ran into it while it was berthed. It was succeeded by the much improved Ictineo II.
A modern replica of the Ictineo I stands in the garden entrance to the Marine Museum in Barcelona.
Four ballast tanks (or bladders, as he called them) were installed in the free-flooding area between the submarine's two hulls, two located forwards and two towards the rear, all controlled from inside the pressure hull using valves to admit water and pumps to force in air. An emergency system was fitted to ensure that the submarine could surface even if the ballast tank system failed, consisting of two sets of large weights fitted to the exterior which could be jettisoned to give positive buoyancy. Monturiol also included a large weight mounted inside the submarine on a longitudinal metal track, which could be slid back and forth to counteract shifts in the vessel's centre of gravity.
For propulsion, Monturiol used hand-cranked propellers. He created a way to remove carbon dioxide from the submarine's interior by forcing air through a container of calcium hydroxide. He also devised a method of producing oxygen which unfortunately proved unfeasible because it produced sulfuric acid. For interior illumination he used a simple candle, which had the advantage of turning red when oxygen began to run low and so alerted the crew.