Prince Nikolai Sergeyevich Trubetzkoy (Moscow, April 15, 1890 - Vienna, June 25, 1938) was a Russian linguist whose teachings formed a nucleus of the Prague School of structural linguistics. He is widely considered to be the founder of morphophonology.

Trubetskoy was born into an extremely refined environment. His father was a first-rank philosopher whose lineage ascended to the medieval rulers of Lithuania. Having graduated from the Moscow University (1913), Trubetskoy delivered lectures there until the revolution. Thereafter he moved first to the university of Rostov-na-Donu, then to the university of Sofia, and finally took the chair of Professor of Slavic Philology at the University of Vienna in 1922. He died from a heart attack attributed to Nazi persecution following his publishing an article highly critical of Hitler's theories.

Trubetskoy's magnum opus, Principles of Phonology, was issued posthumously (in German). In this book he famously defined phoneme as a smallest distinctive unit within the structure of a given language. This work was crucial in establishing phonology as a discipline separate from phonetics. It is sometimes hard to distinguish Trubetskoy's views from those of his friend Roman Jacobson, who should be held responsible for spreading the Prague school views on phonology after Trubetskoy's premature death.