The Sliven Plain is the last of the eastern Sub-Balkan plains; the area geographically called Sredna Gora curves southward in Sliven. Its morphological extensions are the Bakadzhik Mountains, the St. Iliyskiy Hills tributary, the Monastir Hills, and the Sakar Mountains within Bulgaria. Until the first quarter of the 19th century, the Sliven Plain was part of the Rose Valley, but waves of migration from the city following the Russo-Turkish Wars of 1806–1812 and 1828–1829 led to a decline in rose oil production. East of the city lies the Peach Valley, whose enormous fruit masses were renewed in the first decade of the 21st century.

Sliven is 309 km from the capital Sofia, 177 km from Plovdiv, 211 km from Varna, 113 km from Burgas, and 130 km from the borders of the Hellenic Republic and the Republic of Turkey. The route of the Pan-European Transport Corridor 8 is planned to pass through Sliven.

Sliven's location is advantageous due to its location at a crossroads. It is close to the Republic Pass and the Petolachkata road junction connecting Northern and Southern Bulgaria; the connection to the Thrace highway under construction is 15 km south (towards Yambol); and the main road between Burgas and Sofia connects the port of Burgas and Burgas Airport to the central part of the country.

The city lies in the transitional continental climate zone and is geographically located on the Sliven Plain of the Balkans. It has a distinct four-season climate: winters are mild, summers are relatively warm, and autumn is longer than spring. The descending bora winds, which blow with lower temperatures in northern Bulgaria, are characteristic of the city.

Three rivers flow through Sliven: the Asenovska (also known as the Asenovitsa or Korucha), the Manastirska Reka, and the Novoselska Reka. The Manastirska is a tributary of the Novoselska. All of them flow into the Tundzha River and from there into the Aegean Sea. The longest river flowing near Sliven is the Tundzha.