The Royal Sussex Light Infantry Militia, later the 3rd Battalion, Royal Sussex Regiment, was an auxiliary regiment raised in Sussex on the South Coast of England. From its formal creation in 1778 the regiment served in home defence in all of Britain's major wars. It saw active service during the Second Boer War, and trained thousands of reinforcements during World War I. After a shadowy postwar existence it was formally disbanded in 1953

The Sussex Militia was an auxiliary military force in Sussex on the South Coast of England. From their formal organisation as Trained Bands in 1572 they defended the coastline, watched the Spanish Armada and took an active part in the English Civil War. It was the Sussex Militia who captured the Duke of Monmouth after his unsuccessful Rebellion in 1685. After a long hiatus, the Sussex Militia was reformed in 1778 and provided internal security and home defence in all of Britain's major wars thereafter. It eventually became the Royal Sussex Light Infantry Militia (RSLIM) and also formed the Royal Sussex Militia Artillery. After the Cardwell Reforms the RSLIM became a battalion of the Royal Sussex Regiment and saw active service in the Second Boer War. It served as a Special Reserve training unit in World War I. After 1921 the militia had only a shadowy existence until its final abolition in 1953.