Thomas Day (22 June 1748 - 28 September 1789), was a British author. His reputation rests mainly on Sandford and Merton (1783-1789), a book for the young, which, it may seem quaintly didactic and even ridiculous, was at one time considered of educational value as inculcating manliness and independence.

Day was born in London, and was educated at Charterhouse and at Corpus Christi College, Oxford. He became a great admirer of Jean-Jacques Rousseau and his doctrine of the ideal state of nature. Having independent means he devoted himself to a life of study and philanthropy. His views on marriage reveal something of his character. He brought up two foundlings, one of whom he hoped eventually to marry. They were educated on the severest principles, but neither reached the level of stoicism he had been seeking. After several proposals of marriage to other ladies had been rejected, he married an heiress who agreed with his ascetic lifestyle. He finally settled at Ottershaw in Surrey and took to farming on philanthropic principles. He had many curious and impractical theories, among them one that all animals could be managed by kindness. While riding an unbroken colt he was thrown, near Wargrave , and died of his injuries. His poem The Dying Negro , published in 1773, struck the keynote of the anti-slavery movement . It is also obvious from his other works, such as The Devoted Legions (1776) and The Desolation of America (1777), that he strongly sympathized with the Americans during their War of Independence.