| Otto III, Duke of Brunswick-Harburg | |
| Noble Family: | House of Guelph |
| Father: | Otto II, Duke of Brunswick-Harburg |
| Mother: | Hedwig of East Frisia |
| Spouse: | Hedwig of Brunswick-Wolfenbüttel |
| Birth Date: | 20 March 1572 |
| Birth Place: | Harburg |
| Death Place: | Harburg |
Otto III, Duke of Brunswick-Lüneburg, Lord of Harburg (20 March 1572 in Harburg - 4 August 1641 in Harburg) was a titular Duke of Brunswick-Lüneburg and ruler of the apanage Brunswick-Harburg.
Otto was a son of Duke Otto II of Brunswick-Harburg (1528-1603) from his second marriage, with Hedwig (1535-1616), the daughter of Count Enno II of East Frisia.
After the death of his brother Christopher in 1606, Otto III and his brother William Augustus ruled Harburg jointly. Their joint reign was described as harmonious. In a treaty of 11 January 1630, the brothers renounced their right to succeed in Brunswick-Lüneburg, in favour of Christian, in exchange for Christian paying their debts, which exceeded .[1]
On 14 April 1621 in Wolfenbüttel, Otto III married Hedwig (1580-1657), the daughter of Duke Julius of Brunswick-Wolfenbüttel. This marriage remained childless.