The Treaty of Croydon, signed in the spring of 1778 and approved by the Atlantean Assembly shortly thereafter, formally ended the Atlantean Revolutionary War between the Kingdom of Great Britain and the United States of Atlantis, which had rebelled against British rule starting in 1775.

Negotiations took place in the same city as the decisive campaign of the war, conducted only months earlier. General Victor Radcliff spoke for the Atlantean Assembly; Richard Oswald, spokesman for the British Secretary of State the Earl of Shelburne, and David Hartley, Member of Parliament, represented His Majesty's Government. The day after negotiations ended, Isaac Fenner and Custis Cawthorne arrived from Paris to conduct talks, but instead found themselves faced with a fait accompli and a cramped bed in Radcliff's rented room.

The treaty settled boundaries; the USA's only land border was shared with Spanish Atlantis, in the far south of the continent. It also settled fishing rights for both states as well as creditors' claims. The only stumbling ground was the issue over loyalists and their property and rights in the new country. Hartley earnestly recommended to Radcliff that the Atlantean Assembly itself "earnestly recommend" to the parliaments of the states that the restitution of loyalist rights and properties be conducted as fairly and squarely as possible. Fenner had grumblings over this issue, Article V of the final Treaty, but conceded to Radcliff and the Britons in the end.