Rizzo will receive a $1.75 million bonus, according to MLBPipeline.com's Jonathan Mayo, which is above the slot value of $1,252,100 for the No. 50 overall spot.
• 2016 Draft signing and bonus tracker
Rizzo was ranked the No. 40 overall Draft prospect by MLB Pipeline.com, and the Mariners had him rated much higher than that, due to the compact left-handed swing and high baseball IQ that project him as a plus hitter at the Major League level.
Scouts are divided by Rizzo's smaller stature -- he is listed at 5-foot-9, 194 pounds -- and remain unsure if Rizzo will end up staying at third base or transitioning to a corner-outfield spot. But few doubt the South Carolina-commit will hit after he followed his .606 junior season batting average with a .392 mark his senior year at Oakton High School.
The club also announced the signing of third-round pick Bryson Brigman. The University of San Diego shortstop's $700,000 bonus agreement had previously been reported by MLB.com.
New signings announced were Alabama left-hander Thomas Burrows (fourth round), Illinois catcher Jason Goldstein (ninth), Missouri Columbia right-hander Reggie McClain (13th), USC right-hander Kyle Davis (14th), McLennan Community College outfielder Dimas Ojeda (17th), Penn State right-hander Jack Anderson (23rd) and Franklin Pierce University right-hander Paul Covelle (27th).
Rizzo and Ojeda will report to the Arizona Rookie League in Peoria, Ariz., while the others will open the season with Class A Short Season Everett.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.