The Siege of Boston (April 19, 1775 – March 17, 1776) was the opening phase of the American Revolutionary War. In the siege, American patriot militia led by newly-installed Continental Army commander George Washington prevented the British Army, which was garrisoned in Boston, from moving by land. Both sides faced resource, supply, and personnel challenges during the siege. British resupply and reinforcement was limited to sea access, which was impeded by American vessels. The British ultimately abandoned Boston after eleven months, moving their troops and equipment north to Nova Scotia.

The siege began on April 19 after the Revolutionary War's first battles at Lexington and Concord, when Massachusetts militias blocked land access to Boston. The Continental Congress, meeting in Philadelphia, formed the Continental Army from the militias involved in the fighting and appointed George Washington as commander in chief. In June 1775, the British seized Bunker Hill and Breed's Hill, two Continental Army positions overlooking Boston; their casualties proved heavy, and their gains insufficient to break the Continental Army's encirclement. After this, the Americans laid siege to Boston; no major battles were fought during this time, and the conflict was limited to occasional raids, minor skirmishes, and sniper fire. British efforts to supply their troops were significantly impeded by the smaller but more agile Continental Army and patriot forces that were operating on land and sea. The British suffered from a continual lack of food, fuel, and supplies.

In November 1775, George Washington sent Henry Knox on a mission to bring the heavy artillery that had recently been captured at Fort Ticonderoga. In a technically complex and demanding operation, Knox brought the cannons to Boston in January 1776, and this artillery fortified Dorchester Heights which overlooked Boston harbor. This development threatened to cut off the British supply lifeline from the sea. British commander William Howe saw his position as indefensible, and he withdrew his forces from Boston to Halifax, Nova Scotia on March 17.

Before 1775, the British imposed taxes and import duties on the American colonies, to which the Americans objected since they lacked British Parliamentary representation. In response to the Boston Tea Party and other acts of protest, 4,000 British troops were sent to occupy Boston under the command of General Thomas Gage and to pacify the restive Province of Massachusetts Bay. Parliament authorized Gage to disband the government of Massachusetts Bay, led by John Hancock and Samuel Adams, among numerous other powers, but the Americans formed the Massachusetts Provincial Congress and continued to meet. The Provincial Congress called for the organization of local militias and coordinated the accumulation of weapons and other military supplies. Under the terms of the Boston Port Act, Gage closed the Boston port, which caused much unemployment and discontent.

British forces went to seize military supplies from the town of Concord on April 19, 1775, but militia companies from surrounding towns opposed them at the Battles of Lexington and Concord. At Concord, some of the British forces were routed in a confrontation at the North Bridge. The British troops were then engaged in a running battle during their march back to Boston, suffering heavy casualties. All of the New England colonies raised militias in response to this alarm and sent them to Boston.

The British Army order of battle in July 1775 was:

Immediately after the battles of April 19, the Massachusetts militia formed a siege line extending from Chelsea, around the peninsulas of Boston and Charlestown, to Roxbury, effectively surrounding Boston on three sides. The siege line was under the loose leadership of William Heath, who was superseded by General Artemas Ward late on April 20. They particularly blocked the Charlestown Neck, the only land access to Charlestown, and the Boston Neck, the only land access to Boston, which was then a peninsula, leaving the British in control only of the harbor and sea access.

The size of the colonial forces grew in the following days, as militias arrived from New Hampshire, Rhode Island, and Connecticut. General Gage wrote of his surprise at the number of Patriots surrounding the city: "The rebels are not the despicable rabble too many have supposed them to be.... In all their wars against the French they never showed such conduct, attention, and perseverance as they do now."