The Mirna (Italian: Quieto) is a river in Istria, Croatia. In ancient times it was called the Aquilis. It is Istria's longest and richest river, being 53 km (33 mi) long and having a basin covering an area of 458 km (177 sq mi). It rises near Buzet, passes along Motovun and empties into the Adriatic Sea near Novigrad.