The Baltimore riot of 1861 (also called the Pratt Street Riot and the Pratt Street Massacre) was an incident that took place on April 19, 1861 in Baltimore, Maryland between Confederate sympathizers and infantrymen of the United States Army. It is regarded by historians as the first bloodshed of the American Civil War.

On April 12, one week prior to the riot, the battle of Fort Sumter started, signaling the beginning of the American Civil War. At the time, the slave states of Virginia, North Carolina, Tennessee, and Arkansas had not yet seceded from the U.S.. In addition, it was not yet known whether four other slave states, (Delaware, Maryland, Missouri, and Kentucky) (later known as "border states"), would remain in the Union. When Fort Sumter fell on April 13 without a single man lost, the Virginia legislature took up a measure on secession. After little debate, the measure passed on April 17. The other southern states watched with interest to see what would happen, as the secession of Virginia was important because of the state's industrial value. Influential Marylanders who had been supportive of secession ever since John C. Calhoun spoke of "nullification" and agitated to join Virginia in leaving the Union. Their discontent increased in the days afterward while Lincoln put out a call for volunteers to serve 90 days and end the insurrection; newly formed units were starting to transport themselves south. Baltimore was a particularly secession-sympathetic city; Abraham Lincoln received only 1,100 of more than 30,000 votes cast for president in 1860. One regiment of newly called up Union troops came through Baltimore; however, anti-Union forces were too disorganized and surprised to do anything about it. When the next regiment came on April 19, however, they were ready.

On April 19, the Union's Sixth Massachusetts Regiment was traveling south to Washington, D.C. through Baltimore. At that time, there was no direct rail connection between the Philadelphia, Wilmington and Baltimore Railroad's President Street Station (on the northeast side of town nearer to Philadelphia) and the Baltimore and Ohio Railroad's Camden Station (on the southern side of town) due to ordinances prohibiting the use of steam locomotives in the inner city and the lack of union stations at the time. Rail cars that transferred between the two stations had to be pulled by horses along Pratt Street.

As the regiment transferred between stations, a mob of secessionists and Southern sympathizers attacked the train cars and blocked the route. When it became apparent that they could travel by horse no further, the troops got out of the cars and marched in formation through the city. However, the mob followed the soldiers, breaking store windows and causing damage until they finally blocked the soldiers. The mob began throwing paving stones and bricks at the troops. Panicked by the situation, several soldiers fired into the mob, and chaos immediately ensued as a giant brawl began between the soldiers, the violent mob, and the Baltimore police. In the end, the soldiers got to the Camden Station, and the police were able to block the crowd from them. The regiment had left behind much of their equipment, including their marching band.

Four soldiers {Corporal Sumner Needham of Co I; Privates Luther C. Ladd; Charles Taylor; and Addison Whitney of Co D} and twelve civilians were killed in the riot. Sumner Henry Needham is considered to be the first Union casualty of the war, though technically he was killed by civilians in a Union state. Ladd; Taylor and Whitney are buried in Lowell, Massachusetts .

After the April 19th rioting, some small skirmishes occurred throughout Baltimore between citizens and police for the next month, but a sense of normalcy returned as the city was cleaned up. Mayor George William Brown and Maryland Governor Thomas Hicks implored President Lincoln to reroute troops around Baltimore city and through Annapolis to avoid further confrontations. On the evening of April 20th Hicks also authorized Brown to dispatch the Maryland state militia for the purpose of disabling the railroad bridges into the city - an act he would later deny. One of the militia captains was John Merryman, who was arrested without a writ of habeas corpus one month later, sparking the case of Ex parte Merryman.

Lincoln rerouted troops through Union-friendly Annapolis at first. Once enough troops had made it to Washington, D.C. to defend the capital, Lincoln resolved to end the problems in Baltimore and restore the rail connection. On May 13, the Union army entered Baltimore, occupied the city, and declared martial law. The mayor, city council, and police commissioner, who were pro-South and seemingly incompetent at maintaining order in the situation, were arrested and imprisoned at Fort McHenry. Meanwhile, the states of Arkansas and Tennessee, seeing how federal troops acted in the pro-Southern state of Maryland on April 19 seceded on May 6. Other Southerners also reacted with hostility to the battle; James Ryder Randall, a teacher in Louisiana but a native Marylander who had lost a friend in the riots, wrote "Maryland, My Maryland" for the Southern cause in response to the riots. It was a poem later set to music popular in the South referencing the riots with lines such as "Avenge the patriotic gore / That flecked the streets of Baltimore." 78 years later, it would become Maryland's state song, though there have been efforts to remove it since.

After the occupation of the city, Union troops were garrisoned throughout the state. Several members of the Maryland legislature were arrested, days before a delayed secession vote which historians now consider likely to have failed even without those arrests, and the state was placed under direct federal administration. Days afterward, North Carolina became the final state to approve secession (May 21). Delaware was occupied by Union troops due to its proximity to (and to prevent a repeat of the events that took place in) Maryland. Kentucky declared its neutrality (although it would eventually join the Union's side), and although Missouri was on the Union side, a Confederate government-in-exile existed in Arkansas and Texas. Maryland would remain under federal administration until April 1865, the end of the war.