Terms of the deal were not disclosed.
Watson attended Lakewood (Calif.) High School, where he went 4-3 with a 1.19 ERA in 11 games, striking out 79 in just 53 innings. He has a powerful fastball, a buckling curveball and a changeup. He's just started throwing a cutter.
Watson, who originally committed to USC, is expected to open with the Rookie-level Gulf Coast League Phillies.
"He's just very aggressive," Marti Wolever, the Phillies' assistant general manager of amateur scouting, said. "He wears it on his sleeve. He doesn't back down. I think he'll fit in perfectly here."
Quinn Roberts is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.