Union County scored four runs in the home seventh inning for a 5-2, walk-off win over Hartsville Wednesday night.
The Jackets take the title series two games to none and win their third state crown in four years.
Byrnes fell to White Knoll, 4-3 in 5A, forcing a decisive third game Friday night at Lander.
Crescent lost at Hanahan, 2-1, meaning the Tigers and Hawks will play a deciding game Friday at USC.
Dixie built an early 3-0 lead but lost in walk-off fashion at East Clarendon, 4-3, giving the Wolverines the 1A state crown.