Thomas Clarkson was an English abolitionist, and a leading campaigner against the slave trade in the British Empire. He helped found The Society for Effecting the Abolition of the Slave Trade (also known as the Society for the Abolition of the Slave Trade) and helped achieve passage of the Slave Trade Act 1807, which ended British trade in slaves.

He became a pacifist in 1816 and, together with his brother John, was among the twelve founders of the Society for the Promotion of Permanent and Universal Peace.

In his later years, Clarkson campaigned for the abolition of slavery worldwide; it was then concentrated in the Americas. In 1840, he was the key speaker at the Anti-Slavery Society's (today known as Anti-Slavery International) first conference in London which campaigned to end slavery in other countries.