On Thursday, the club announced that it has signed the University of Washington product to a contract.
Hagadone will report to Class A Lowell of the New York-Penn League immediately.
A college teammate of Giants starter Tim Lincecum, Hagadone has a low- to mid-90s fastball and a plus-slider. He was first-team All-Pac 10 in 2007, going 6-1 with 11 saves and a 2.77 ERA in 25 games, two of which were starts. A closer this season, Hagadone projects as a starter.
The Red Sox have now signed 19 players from the First-Year Player Draft.
Overall, it was a good day for the Boston scouting department. The Red Sox also announced the signing of eight international free agents from four different countries.
That group is headlined by Michael Almanzar, a 16-year-old third baseman from the Dominican Republic.
"Of course Michael is the marquee name on the list," said Craig Shipley, vice president/international scouting for the Red Sox. "He's a player I think that a lot of teams had a lot of interest in. The market is becoming increasingly competitive. We identified him at a very young age. We've been scouting him for a long time.
"Mike is a player who has a very projectable body, projectable tools. He has the blood lines [father Carlos pitched in big leagues]. We're very happy to have him."
The other international signings are as follows: Right-hander Justin Erasmus (Australia), shortstop Rafael Espinosa (Dominican Republic), shortstop Joantoni Garcia (Venezuela), outfielder Javier Gutierrez (Venezuela), outfielder Che-Hsuan Lin (Chinese Taipei), right-hander Roman Mendez (Dominican Republic) and catcher Jesus Rojas (Venezuela).
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.