Powder House Square is a neighborhood and landmark rotary in Somerville, Massachusetts, United States. It is also known locally as Powder House Circle. It is the six-way intersection of College Avenue, Broadway, Warner Street, and Powder House Boulevard. Powder House Square stands at the southern tip of Tufts University's main Somerville/Medford campus, and borders the northern edge of Nathan Tufts Park. The square takes its name from the 18th century Powder House, which overlooks the rotary from Nathan Tufts Park.

Powder House Square was home to one of the first hostile acts of the American Revolution. The removal of colonial gunpowder by British soldiers, and the massive popular reaction known as Powder Alarm that ensued, are considered to be a turning point in the events leading up to war.

Broadway was one of the earliest highways running through what would later become Somerville, originating in the 17th century. Originally called "Menotomie's Road," it ran from Charlestown to the settlement at Menotomy (present-day Arlington).

Powder House Boulevard was created in 1900, extending northwest from Broadway at Nathan Tufts Park. According to a plaque placed at the center of the rotary island, the circle was dedicated in 1940 as the James A. Reynolds Traffic Circle, in honor of a Tufts College professor.

First built for use as a windmill by John Mallet in the early 1703 or 1704, the Old Powder House was sold to the colonial government of Massachusetts for use as a gunpowder magazine in 1747. It is the oldest stone building in Massachusetts. Located at the intersection of Broadway and College Avenue in present-day Powder House Square, the Old Powder House held the largest supply of gunpowder in the colony. General Thomas Gage, who had become the military governor of Massachusetts in May 1774, was charged with enforcement of the highly unpopular Intolerable Acts, which the British Parliament had passed in response to the Boston Tea Party. Seeking to prevent the outbreak of war, he believed that the best way to accomplish this was by secretly removing military stores from storehouses and arsenals in New England.

Just after dawn on September 1, 1774, a force of roughly 260 British regulars from the 4th Regiment, under the command of Lieutenant Colonel George Maddison, were rowed in secrecy up the Mystic River from Boston to a landing point near Winter Hill. From there they marched about a mile to the Powder House, and after sunrise removed all of the remaining gunpowder, all of which was British-owned. Most of the regulars then returned to Boston the way they had come, but a small contingent marched on to Cambridge, seizing two field pieces from the Cambridge Common. The field pieces and powder were then taken from Boston to the British stronghold on Castle William.

In response to the raid, amid false rumors that blood had been shed, alarm spread through the countryside as far as Connecticut and beyond, and local Patriots formed themselves into armed mobs, fearing that war was at hand. Thousands of militiamen began streaming toward Boston and Cambridge, and mob action forced Loyalists and some government officials to flee to the protection of the British Army. This action provided a "dress rehearsal" for the Battles of Lexington and Concord seven months later in the famous "shot heard 'round the world". This did however, inflame already heated feelings on both sides and would spur actions by both the British and Patriots to move both powder and cannons to secure locations.

Massachusetts began storing gunpowder elsewhere and sold the land containing the Powder House to Peter Tufts in 1818. The estate, known as the Powder House Farm, was used as a farm and residence for the Tufts family for most of the 19th century, passing to Peter's heir, Nathan Tufts. Incidentally, another Tufts relative Charles Tufts donated the land across Broadway on the Somerville/Medford line to found Tufts College (now Tufts University). The college was founded in 1852, on Walnut Hill slightly to the north of present-day Powder House Square.