狄奥多西一世（Theodosius I，347年－395年1月17日）又译为狄奥西亚一世或杜多思一世，亦作狄奥多西大帝，是罗马帝国皇帝（379年－395年），392年起统治整个罗马帝国。他是最后一位统治统一的罗马帝国的君主，并将基督教定为国教。

狄奥多西一世本名弗拉维乌斯·狄奥多西（Flavius Theodosius），出生于现今西班牙塞维亚的基督教信徒家庭并随父狄奥多西从军。在368年父子一同在不列颠尼亚讨平由于外族人的入侵和部分驻守哈德良长城的士卒叛变而酿成的一年混乱局面。混乱平定后他们进行了一些军事和民政的改革，但当地日益衰败的罗马帝国统治并无明显起色。374年，狄奥多西一世已为莫西亚的军事长官。其后不久父亲被逮捕并于375年初被处决，狄奥多西本人返回出生地。这次包括本人失去军职和父亲被杀的原因和其中可能的关系并不清楚。由于狄奥多西一世部队中两支军团在374年底被萨尔马特人击败，他可能因而被皇帝瓦伦提尼安一世剥夺指挥权。父亲狄奥多西干预皇帝的决定而被杀。

Theodosius I (Latin:Flavius Theodosius Augustus; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman Emperor from AD 379 to AD 395. Theodosius was the last emperor to rule over both the eastern and the western halves of the Roman Empire. On accepting his elevation, he campaigned against Goths and other barbarians who had invaded the empire. He failed to kill, expel, or entirely subjugate them, and after the Gothic War, they established a homeland south of the Danube, in Illyricum, within the empire's borders. He fought two destructive civil wars, in which he defeated the usurpers Magnus Maximus and Eugenius at great cost to the power of the empire.