The Rusokastrenska reka (Bulgarian: Русокастренска река) is a 65 km long river in eastern Bulgaria that flows into Lake Burgas, which drains into the Black Sea.

[1] In 1332 near the river's left banks close to the fortress of Rusokastro the Bulgarian emperor Ivan Alexander defeated the Byzantine army under emperor Andronikos III Palaiologos in the battle of Rusokastro.

[2] The river takes its source from a spring on the southern slopes of the Hisar Heights at an altitude of 335 m, some 2.4 km southeast of the town of Karnobat.

It flows in southeastern direction through the Burgas Plain in a wide valley covered with farmlands.

At the village of Zhelyazovo it bends north to bypass the isolated height of Rusen Kamal (108 m), takes its largest tributary the Hadzhilarska reka, and continues southeast.