The Treaty of Neuilly-sur-Seine (French: Traité de Neuilly-sur-Seine; Bulgarian: Ньойски договор) was a treaty between the victorious Allies of World War I on the one hand, and Bulgaria, one of the defeated Central Powers in World War I, on the other.

The treaty was signed on 27 November 1919, in Neuilly-sur-Seine in the Hauts-de-Seine department, just west of Paris in France.

During World War II, together with Nazi Germany, it temporarily reoccupied most of the other territories ceded under the treaty.

Bulgaria was internationally recognised as an independent country in 1908 and controlled these territories until 1919 when they were ceded to the Kingdom of the Serbs, Croats and Slovenes under the Treaty of Neuilly.

The old political boundary between Bulgaria and Serbia followed a chain of high mountain ridges, whereas the new one gave significant military and strategic advantages to the Serbs: it dangerously exposed the Bulgarian capital of Sofia and significantly reduced the military threat to eastern Serbia in case of a Bulgarian invasion (see also Balkan Wars and World War I).