Jackson was the sixth overall pick in the Draft, a spot that carries a $3.57 million value in MLB's current slotting system. The Mariners had room to go up to about $4.2 million without going over their cumulative signing bonus limit, and Jackson is believed to have signed for right in that neighborhood.
Teams that exceed their bonus-pool limits by 5 percent or less are subject to a 75 percent tax on that money, and anything above 5 percent results in the loss of future first-round Draft picks.
Jackson is being listed as an outfielder by the Mariners, though he played mostly catcher at Rancho Bernardo High School in San Diego. The 18-year-old was regarded by many as the top power hitter in this year's Draft.
Mariners general manager Jack Zduriencik has said Jackson is a good athlete who could be a quality catcher, but with 2012 first-round Draft pick Mike Zunino already at that position, Seattle will start Jackson in the outfield and see how quickly he can advance.
The 6-foot-2, 215-pounder hit 47 home runs in his prep career at Rancho Bernardo, a traditional baseball power in Southern California.
With Jackson's agreement, the Mariners have now signed their top 14 picks.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.