The Pirinska Bistritsa or Mpístritsas (Bulgarian: Пиринска Бистрица, Greek: Μπίστριτσας) is a river in south-western Bulgaria and northern Greece, a left tributary of the Struma.

Near Pirin refuge it turns to the southeast and enters a densely forested area, where it again flows southwards.

At the village of Pirin the river takes a sharp turn to the southwest and enters a sparsely forested valley with eroded slopes.

From that point it forms the border between Bulgaria and Greece until its confluence with the Struma at an altitude of 69 m between the villages of Kulata and Promachonas.

[1] There are four settlements along the river, all of them situated in Sandanski Municipality: Pirin, Gorno Spanchevo, Katuntsi and Novo Hodzhovo.