Sir William Gore Ouseley (26 July 1797 – 6 March 1866) was a British diplomat who served in various roles in Washington, D.C., Rio de Janeiro and Buenos Aires.

His diplomatic activity in Costa Rica was favored by the fact that he was a relative of Sophia Joy, the British second wife of President José María Montealegre (1859-1863).

There he fortunately arrived at a time when the palace was so infested by mice or rats, that they invaded the king's food, and persons were employed to drive them from the royal banquet.

At that time there was a proposal to withdraw the British naval squadrons that patrolled the Atlantic to put down the slave trade on the grounds that they were ineffective and cost too much money.

Appealing to his own experience as a diplomat in Brazil, he recalled that by "the active exertions of our cruisers and the intelligent and zealous co-operation of our officers" the slave trade from Africa to Brazil was greatly reduced.Nearly a score of slave-vessels lay idle in the harbour of Rio de Janeiro, the owners, masters, and crews, finding the risks too great and of too disagreeable a nature, would not venture out.Ouseley argued against slavery on moral and religious grounds, and said it was exacerbated by racial prejudice.