This stately building left unfinished (1795) was rebuilt by architect Giuseppe Achilli.
In the site of today’s Palazzo Diotallevi there was a prestigious Roman house, discovered in 1975 and since then known as the large domus of Palazzo Diotallevi. 
The oldest evidence dates back to the period of the foundation of the colony of Ariminum. The house plan can be dated to the late 2nd – early 1st century B.C.  Rebuilt in the 2nd century A.D. the house was transformed into an elegant Domus with rooms giving onto a large T shaped courtyard and a large room with attractive decoration highlighting a mosaic with a harbour scene: the so-called Mosaic of the Boats, showing a merchant convoy landing. The host's profession is alluded to in this scene so he could show his success in the world of sea trade.
The striking mosaic with port scene is now preserved in the City Museum.