The Battle of Gaines' Mill, sometimes known as the Battle of Chickahominy River, took place on June 27, 1862, in Hanover County, Virginia, as the third of the Seven Days Battles which together decided the outcome of the Union's Peninsula Campaign of the American Civil War. Following the inconclusive Battle of Beaver Dam Creek (Mechanicsville) the previous day, Confederate General Robert E. Lee renewed his attacks against the right flank of the Union Army, relatively isolated on the northern side of the Chickahominy River. There, Brig. Gen. Fitz John Porter's V Corps had established a strong defensive line behind Boatswain's Swamp. Lee's force was destined to launch the largest Confederate attack of the war, about 57,000 men in six divisions. Porter's reinforced V Corps held fast for the afternoon as the Confederates attacked in a disjointed manner, first with the division of Maj. Gen. A.P. Hill, then Maj. Gen. Richard S. Ewell, suffering heavy casualties. The arrival of Maj. Gen. Stonewall Jackson's command was delayed, preventing the full concentration of Confederate force before Porter received some reinforcements from the VI Corps.

The Seven Days Battles were a series of seven battles over seven days from June 25 to July 1, 1862, near Richmond, Virginia, during the American Civil War. Confederate General Robert E. Lee drove the invading Union Army of the Potomac, commanded by Maj. Gen. George B. McClellan, away from Richmond and into a retreat down the Virginia Peninsula. The series of battles is sometimes known erroneously as the Seven Days Campaign, but it was actually the culmination of the Peninsula Campaign, not a separate campaign in its own right.