Sliven (Bulgarian: Сливен [ˈslivɛn]) is the eighth-largest city in Bulgaria and the administrative and industrial centre of Sliven Province and municipality in Northern Thrace. It is situated in the Sliven Valley at the foothills of the Balkan Mountains.

Sliven is famous for its heroic Haiduts who fought against the Ottoman Turks in the 19th century and is known as the "City of the 100 Voyvodi", a Voyvoda being a leader of Haiduts.

The famous rocky massif Sinite Kamani (Сините камъни, "The Blue Rocks") and the associated nature park, the fresh air and the mineral springs at Slivenski Mineralni Bani offer diverse opportunities for leisure and tourism. Investors are exploring the opportunity to use the local wind (Bora) for the production of electricity.

Another point of interest and a major symbol of the city as featured on the coat of arms, is the more than thousand-year-old Stariyat Briast (Старият Бряст, "The Old Elm"), a huge Smooth-leaved Elm in the center of the city. During Ottoman rule, Turkish officials used to hang Bulgarian revolutionaries on it. Today the city is helping the tree to live on by frequent evaluations and reinforcing its base. It was elected Bulgarian tree of the year in 2013. On 19 March 2014 the results of an online poll were revealed at a ceremony in the European Parliament. The Old Elm was voted European Tree of the Year 2014. The city also served as an important strategic centre for the Bulgarian Army, with the headquarters of the Bulgarian Third Army located in the centre of the city, being situated relatively near to the sensitive Turkish border.

Sliven Peak on Livingston Island in the South Shetland Islands, Antarctica is named after Sliven.