The Blatnitsa (Bulgarian: Блатница) is a river in southern Bulgaria, a left tributary of the river Sazliyka, itself a left tributary of the Maritsa. It has a length of 54 km.^[1]

The river takes its source at an altitude of 266 m from the Kapaklia spring, located in the easternmost reaches of the Sredna Gora mountain range, some 1 km southeast of the village of Staro Selo. Throughout its whole course the Blatnitsa flows through the Upper Thracian Plain in a shallow valley. It initially flows south, at the village of Konyovo it turns in direction west-southwest until the town of Nova Zagora, when it takes a turn to the south-southwest until its confluence with the Sazliyka at an altitude of 108 m in the southwestern neighbourhoods of the town of Radnevo.^[1]

Its drainage basin covers a territory of 656 km² or 20.25% of the Sazliyka's total.^[1]

The Blatnitsa has predominantly rain feed with high water in January–May and low water in July–October.^[1]

The river flows in Sliven and Stara Zagora Provinces. There are five settlements along its course: the villages of Konyovo, Ezero, Bogdanovo and Lyubenova Mahala in Nova Zagora Municipality of Sliven Province, and the town of Radnevo in Radnevo Municipality of Stara Zagora Province. A 23.7 km stretch of the third class III-554 road Nova Zagora–Radnevo–Harmanli follows its valley.^[2] Along the same stretch passes a section of the Simeonovgrad–Nova Zagora railway. Its waters are almost completely utilised for irrigation for the intensive agriculture in the Upper Thracian Plain.^[1]