Osimo is a town and comune of the Marche, Italy, in the province of Ancona, 15 km south of that town by rail. It is situated on a hill near the Adriatic Sea. Vetus Auximum was founded by the same Greek colonists of Ancona; later it was conteded between the Gauls and the Piceni, until conquered by the Romans, who used it as a fortress for their northern Picenum settlement starting from 174 BC.
The walls were made of large rectangular stones which are still there in some part. It was a colony until 157 BC. The family of Pompey were its protectors and resisted Caesar in 49 BC. It is considered to have been important during imperial times due to inscriptions and monuments in its town square.
Osimo retains a portion of its ancient town wall (2nd century BCE).
The restored Romanesque-Gothic cathedral (8th-12th centuries) has a portal with sculptures of the 13th century, an old crypt, a fine bronze font of the 16th century and a series of portraits of all the bishops of the old diocese of Osimo. The baptistery is from the early 17th century and also has a notable baptismal font.
The town hall contains a number of statues found on the site of the ancient forum. The second oldest church in Osimo is that of San Giuseppe da Copertino (Saint Joseph of Cupertino), built in the 13th century.
Under the town is a large series of tunnels with esoteric bas-reliefs.
The new castle (1489), of which parts remain today, was built by Baccio Pontelli.