Stephenson was drafted 11th overall, a pick that has a slot value of $3,141,600. The Reds did not disclose terms of the deal.
"It's everything you can possibly imagine," Stephenson said on Monday after he was drafted. "The Reds, you can't beat it. Great ballpark. Great city. ... It's a dream come true."
Stephenson, 18, was ranked by MLB.com as the best catcher in this year's Draft class. He hit .415 with eight home runs and 25 RBIs this season for Kennesaw Mountain High School in Marietta, Ga.
The Reds have assigned Stephenson to the rookie Pioneer League's Billings roster. He had been committed to play baseball at Georgia Tech.
Austin Laymance is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.