The Sirena class was an improved and enlarged version of the preceding Argonauta-class submarines.

[1] For surface running, the boats were powered by two 675-brake-horsepower (503 kW) diesel engines, each driving one propeller shaft.

They were also armed with a single 100 mm (3.9 in) deck gun forward of the conning tower for combat on the surface.

[1] Topazio was laid down by Cantieri navali del Quarnaro at their Fiume shipyard in 1931, launched on 15 May 1933 and completed the following year.

[1] During the Spanish Civil War, she bombarded Valencia with 34 shells on the night of 12/13 February 1937.