The Treaty of Constantinople was a treaty between the Ottoman Empire and the Kingdom of Bulgaria signed on 29 September 1913 after the Second Balkan War at the Ottoman capital İstanbul.
In the First Balkan War, the coalition of Bulgaria, Serbia, Greece and Montenegro along with Albanian rebels defeated the Ottoman Empire. The Ottomans lost nearly all their European possessions, save for a small territory around the Sea of Marmara by the Treaty of London. The Ottomans however were able to recover Eastern Thrace during the Second Balkan War. Although peace talks between Bulgaria and her other neighbours were held in Bucharest, the Ottoman Empire was not represented there and conducted separate negotiations which led to the Treaty of Constantinople.
 The treaty
The terms of the treaty were: 
- Bulgaria acknowledged Ottoman gains of Edirne, Kırklareli and Demotika (Didymoteicho) and the surrounding territory
- The Ottoman Empire ceded the port of Dedeagach (mod. Alexandroupoli) to Bulgaria
- The exchange of lands was to be completed within 10 days
- The armies on the border would be demobilized within three weeks
- Prisoners of war from both sides would be released
- Both political and economical ties between the two countries would be reestablished
The Ottoman Empire and Bulgaria were the allies in the Central Powers in the First World War. During the war, the Ottoman government decided to cede Didymoteicho to Bulgaria (probably to persuade Bulgaria to join the war). However the Central Powers were defeated in 1918 and Bulgaria lost both the Western Thrace and Didymoteicho to Greece.