The Ausa is a minor river some 17 kilometres (11 mi) long that traverses part of northern San Marino and Emilia–Romagna in Italy.

The river flows northeast past Serravalle and crosses the border into the Italian province of Rimini close to Dogana.

It flows northeast through the towns of Serravalle and Dogana, and forms the northern border between Italy and San Marino as it curves east past Rovereta.

[2] The Ausa, known to the ancient Romans as the Aprusa, formed the southern border of the colonia of Ariminum, but was too small to provide a natural defence, and so was supplemented by Rimini's city walls.

[2] In the late 19th century, the Ausa divided Rimini's coastline between the elegant bathing establishment to its north and the unattended beach to its south.

[2] The diversion was complete in 1972, with the Ausa's former route reduced to a sewage outlet,[3][8] and redeveloped into a series of public parks.