Thomas Sim Lee (October 29, 1745 – November 9, 1819) was an American planter and politician who served as Maryland Governor for five one-year terms (1779-1783 and 1792-1794), as well as in the Congress of the Confederation (1783–84), Maryland Ratification Convention of 1788 and House of Delegates in 1787. He also held local offices and owned many town lots in Georgetown (which became part of the new federal city, Washington, District of Columbia, and spent his final decades operating "Needwood" plantation in Frederick County, Maryland. In addition to working closely with many of the Founding fathers, he played an important part in the birth of his state and the nation.

Lee was born in 1745 in Upper Marlboro in the Province of Maryland. He was the son of Thomas (died 1749) and Christiana (Sim) Lee, a grandson of Philip Lee, and descended from the "Blenheim" Line of the Lee family of Virginia. British merchant and Virginia politician Richard Lee I was his great-great-grandfather. He received a private education as befit his class, and in 1769 and 1770 was educated in Bath, England.

Lee was a member of the provincial council by 1777. During the Revolutionary War, he backed the patriot cause, and organized a local militia in which he served as colonel

Governor Lee's wife, Mary Digges Lee, also rallied the women of Maryland to raise money in support of the war effort. She then established a correspondence with General George Washington, asking how these resources could be put to best use. General Washington responded suggesting that the money raised be put toward the purchase of much-needed shirts and black neck clothes for the troops in the Southern army. He expressed gratitude to Mrs. Lee for the "patriotic exertions of the ladies of Maryland in favor of the army".

Thomas Sim Lee participated in the Annapolis Convention in the mid-1770s, which produced a constitution for Maryland and transformed the colony into a state. On July 26, 1775, he was one of the signatories of the Declaration of the Association of the Freemen of Maryland, an influential statement in the Revolutionary War.

Lee won his first state elective office in 1777, and served for two years in the Maryland Legislature.

Fellow Maryland legislators elected Lee governor in 1779. He was reelected in 1780 and 1781. During his first tenure, issues regarding the war effort were dealt with. He won wide praise for his logistical abilities as governor. Lee consistently procured fresh troops and supplies for the Continental Army. George Washington was Lee's friend, and learning of the plan to pin down Cornwallis, Lee exerted all his energies to support the American troops. After completing his term, Lee was forbidden to stand for re-election, and so left office on November 22, 1782.

As Governor of Maryland, Thomas Sim Lee signed the Act on February 2, 1781, whereby the Maryland Legislature ratified the Articles of Confederation and Perpetual Union. As Maryland was the 13th and final state to ratify the Articles, the act established the requisite unanimous consent for the formation of a Perpetual Union of the states. Maryland had previously held out and refused to ratify the Articles until every state had ceded its western land claims. After Governor Thomas Jefferson signed the Act of the Virginia legislature on January 2, 1781, to grant these concessions the way forward for Maryland was cleared. On this second day of February, a Friday, as the last piece of business during the afternoon Session, "among engrossed Bills" was "signed and sealed by the Governor, in the Senate Chamber, in the presence of the members of both Houses...an Act to empower the delegates of this state in Congress to subscribe and ratify the articles of confederation." The Senate then adjourned "to the first Monday in August next". The formal signing of the Articles by the Maryland delegates took place in Philadelphia at noon time on March 1, 1781. With these events, the Articles entered into force and the United States came into being as a united and sovereign nation.