The Battle of Mechanicsville also known as the Battle of Beaver Dam Creek,  or Ellerson's Mill, took place on June 26, 1862, in Hanover County, Virginia, as the first major engagement of the Seven Days Battles during the Peninsula Campaign of the American Civil War. It was the start of Confederate General Robert E. Lee's counter-offensive against the Union Army of the Potomac, under Maj. Gen. George B. McClellan, which threatened the Confederate capital of Richmond. Lee attempted to turn the Union right flank, north of the Chickahominy River, with troops under Maj. Gen. Thomas J. "Stonewall" Jackson, but Jackson failed to arrive on time. Instead, Maj. Gen. A.P. Hill threw his division, reinforced by one of Maj. Gen. D.H. Hill's brigades, into a series of futile assaults against Brig. Gen. Fitz John Porter's V Corps, which occupied defensive works behind Beaver Dam Creek. Confederate attacks were driven back with heavy casualties. Porter withdrew his corps safely to Gaines Mill.

Overall, the battle was a Union tactical victory, in which the Confederates suffered heavy casualties and achieved none of their specific objectives due to the seriously flawed execution of Lee's plan. Instead of over 60,000 men crushing the enemy's flank, only five brigades, about 15,000 men, had seen action. Their losses were 1,484 versus Porter's 361. Lee's staff recalled that he was "deeply, bitterly disappointed" by Jackson's performance, but communication breakdowns, poorly written orders from Lee, and bad judgment by most of Lee's other subordinates were also to blame.

Despite the Union tactical success, however, it was the start of a strategic debacle and the unraveling of the Peninsula Campaign. McClellan began to withdraw his army to the southeast and never regained the initiative. The next day the Seven Days Battles continued as Lee attacked Porter at the Battle of Gaines's Mill.