The siege of Augusta took place between May 22, 1781, and June 6, 1781. American Patriot forces, led by Brigadier General Andrew Pickens and Lieutenant Colonel Henry "Light Horse Harry" Lee, were successful in capturing Augusta, Georgia held by British loyalist militia. The "third battle" or siege of Augusta during the Revolutionary War refers to the final engagement, where Patriot forces under Andrew Pickens and "Light Horse Harry" Lee captured the British-held Fort Cornwallis in Augusta, Georgia, on June 5, 1781, after a siege that began in May. The final battle focused on the capture of Fort Cornwallis, where the Patriots erected a 30-foot tower (also known as the Mayham Tower) to fire a cannon into the British-held fort. The Patriots used a siege tower, named after Major Hezekiah Mayham, who used a similar strategy against the British in South Carolina, to fire down on the fortress. Facing overwhelming firepower, the British, led by Lieutenant Colonel Thomas Brown, surrendered on June 5, 1781. On May 21, the stockaded house of George Galphin, an Indian agent, located 12 miles south of Augusta, was attacked by forces under Clarke and Lee. After a brief exchange, in which one Patriot died of the heat and eight to ten were wounded, the British garrison there surrendered after three or four men were killed.

A total of 126 men, mostly regulars, surrendered. The prize was much-needed supplies and military equipment which had been intended for distribution to the local Indians. Fort Grierson was a secondary fortified outpost located about half a mile from Fort Cornwallis. This fort was defended by about 80 men under Colonel Grierson. On May 23, the Patriot forces began to encircle the fort in a manner intended to draw Grierson out in an attempt to reach Fort Cornwallis. Brown, aware of the danger to Grierson, sallied forth from Cornwallis, but when faced with Lee's strength, limited his support attempt to an ineffective cannonade. Grierson, desperate to escape the trap, attempted to flee along the riverbank but his entire company was captured. Clarke's men then took their revenge for actions perpetrated by Brown and refused quarter, killing Grierson and all of his men. On May 23, the main body of Lee's forces arrived in Augusta. A half-mile west of Fort Grierson was Fort Grierson. It was quickly overrun by the Americans. The capture of Augusta was a significant victory for the Patriots, demonstrating their growing strength in the South and giving American peace negotiators in Paris reason to demand Georgia's independence.