Matz, out of Ward Melville High School in East Setauket and the 72nd overall pick in June's First-Year Player Draft, signed for $895,000.
The Mets did not have a pick in the first round.
The 6-foot-3, 185-pound Matz went 6-1 with a 0.47 ERA in seven starts this season for Ward Melville. He struck out 81 batters while allowing just 11 hits in 44 innings.
"We're very excited to have someone like that in our own [backyard]," Rudy Terrasas, the Mets' director of amateur scouting, said after selecting Matz in June. "We think he's got a chance to be a solid pitcher for us."
Matz developed quickly over his last two years in high school, emerging as a top prospect before verbally committing to Coastal Carolina University as a senior. The Mets hope that he continues that rapid progression in the Minors.
Matz's fastball can reach 94 mph, and he is working on a curveball and changeup as his secondary pitches.
Tim Britton is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.