SM U-42 was a submarine ordered for construction by the Imperial German Navy.

The vessel was designed by the naval engineer Cesare Laurenti,[1] and laid down on 18 August 1913 in Italy at Società FIAT-San Giorgio, Muggiano, La Spezia.

The vessel was renamed Balilla and served with the Royal Italian Navy (Regia Marina) until sunk in a battle on 14 July 1916 with two torpedo boats.

On 14 July 1916 she was attacked by two Kaiman-class torpedo boats, the Hydra and Skorpion.

After a lengthy surface battle, Balilla was sunk by gunfire with all hands lost.