The device was found within the remains of one of the 37 ships unearthed in the Yenikapi area of Istanbul, the Turkish capital.
Also known as Theodosius Port, it was built in the late 4th century during the reign of the Byzantine Emperor Theodosius I and become the city's most important commercial port.
The wooden object, whose cover is finely carved with decorations, is the size of a modern seven-inch tablet, but much thicker, and probably belonged to the ship's captain.
It consists of a set of five overlaid rectangular panels carved with frames and covered with wax. Notes could be taken on those panels, as shown by writing in Greek which is still visible on the wax.
A primitive "app" is hidden on the bottom panel: a sliding lid revealing a hidden plate with carved spaces, 'Discovery News' reported.
"When you draw the sliding part, there are small weights used as an assay balance," Ufuk Kocabas,, director of Istanbul University's department of marine archeology and the Yenikapi Shipwrecks Project, was quoted as saying by 'Hurriyet Daily News'.
The sunken ship upon which the device was found, has been dated to around the 9th century AD.
The containers it had been carrying suggest the vessel sailed the Black Sea, transporting goods from Crimea to Kersonesos.