The Ausa River is a minor river some 17 kilometres in length which traverses part of northern San Marino and Emilia–Romagna in Italy. The source of the river is Monte Titano in central San Marino. The river flows northeast past Serravalle and crosses the border into the Italian province of Rimini close to Dogana. The river continues flowing northeast and is channelled through storm drains at Rimini before emptying into the Adriatic Sea south of the mouth of the Marecchia River. At 55 m above sea level, it is the lowest point of San Marino.