The Allegheny Arsenal, established in 1814, was an important supply and manufacturing center for the Union Army during the American Civil War, and the site of the single largest civilian disaster during the war. It was located in the community of Lawrenceville, Pennsylvania, which was annexed by the city of Pittsburgh in 1868.

Today, the site is the location of the nine-acre Arsenal Park as well as Arsenal Middle School, a county health services complex, and a large condominium development.

In 2012, officials from the Pittsburgh History & Landmarks Foundation began drawing attention to the deteriorating arsenal structures.

The Arsenal was established by the U.S. Army Ordnance Department near Pittsburgh in 1814. It was situated on 30 acres (12 ha) of land bordering the Allegheny River in the community of Lawrenceville. The site was bounded by 39th Street, 40th Street, and Penn Avenue, and bisected by Butler Street, which was and still is the main thoroughfare of Lawrenceville.

The arsenal served as a supply and manufacturing center for the troops in the west. Its peak years came during the Civil War, especially when the manufacture of paper cartridges for rifles became a high priority. Civilian employment at the arsenal increased from a pre-war total of 308 to over 1100 workers. One of the busiest facilities was the main lab, which employed 158 workers, the majority of whom were women engaged in the making of cartridges.

On Wednesday, September 17, 1862, around 2 pm, the arsenal exploded. The explosion shattered windows in the surrounding community and was heard in Pittsburgh, over two miles (3 km) away. At the sound of the first explosion, Col. John Symington, Commander of the Arsenal, rushed from his quarters and made his way up the hillside to the lab. As he approached, he heard the sound of a second explosion, followed by a third. Fire fighting equipment as well as a bucket brigade tried to douse the flames with water. The volunteer fire company from Pittsburgh arrived and assisted in bringing the fire under control.

By the time the fire was put out, the lab had been reduced to a pile of smoldering rubble. Seventy-eight workers, mostly young women, were killed. Fifty-four bodies were unidentified, and were buried in a mass grave in the nearby Allegheny Cemetery. Among those killed were 15-year-old munitions assembler Catherine Burkhart, who lived at 184 38th Street, and 17-year-old Margaret Turney, who lived at 160 43rd Street.

The most commonly held view of the cause of the explosion was that the metal shoe of a horse had struck a spark which touched off loose powder in the roadway near the lab, which then traveled up onto the porch where it set off several barrels of gunpowder. A coroner's jury held that the accident had been the result of the negligent conduct of Col. John Symington and his subordinates in allowing loose powder to accumulate on the roadway and elsewhere. However, during a subsequent military inquiry into the conduct of Col. Symington, many of the same witnesses who had appeared before the coroner changed their testimony. There were so many discrepancies between the two hearings that most of the commonly held views of the explosion have been shown to be thoroughly discredited. In the end Col. Symington was found innocent of any wrongdoing by the army, and the court concluded that "the cause of the explosion could not be satisfactorily ascertained...."