According to 1 Samuel, David used a sling against Goliath.
Before the confrontation, David refuses King Saul’s armor because he has not tested it. Instead, he takes his staff, chooses five smooth stones from a stream, and carries his sling. The narrative says that David strikes Goliath in the forehead with one stone, causing the Philistine champion to fall.
David then uses Goliath’s own sword to kill him, which creates a common point of confusion. The sling is the weapon that brings Goliath down; the sword is used afterward. The story therefore supports the short answer “a sling,” not “a sword.”
The account emphasizes the contrast between the heavily armed Goliath and the younger David, who presents himself as trusting Israel’s God rather than conventional armor. Slingers were real military specialists in the ancient Near East, and a sling could launch a stone with considerable force. The five stones are mentioned, but only one is used in the initial strike.