Otto IV (1175–1218) was Holy Roman Emperor from 1209 until his deposition in 1215. A member of the Welf dynasty, he was the son of Matilda, daughter of Henry II, and was raised in the courts of his uncles Richard I and John. In 1190 he was created Duke of York.

Crowned emperor by Pope Innocent III ⓘ in 1209, Otto soon came into conflict with the papacy. In 1210 he was excommunicated after reneging on promises concerning papal territorial claims in Italy and Sicily. Allied with King John of England ⓘ, he invaded France in 1214 but suffered a decisive defeat by Philip II at the Battle of Bouvines, a turning point that fatally weakened his position.

Otto was deposed as emperor in 1215 and spent his final years in relative obscurity. He died at Harzburg Castle on 19 May 1218.