Otto IV (1175 – 19 May 1218) was the Holy Roman Emperor from 1209 until his death.

Otto spent most of his early life in England and France. He was a follower of Richard the Lionheart, who made him Count of Poitou in 1196. With Richard's support, he was elected King of Germany by one faction in a disputed election in 1198, sparking ten years of civil war. The death of his rival, Philip of Swabia, in 1208 left him sole king of Germany.

In 1209, Otto marched to Italy to be crowned emperor by Pope Innocent III. In 1210, he sought to unite the Kingdom of Sicily with the Empire, breaking with Innocent, who excommunicated him. He allied with England against France and took part in the alliance's defeat at Bouvines in 1214. He was abandoned by most of his supporters in 1215 and lived the rest of his life in retirement on his estates near Brunswick. He was the only German king of the Welf dynasty.