Castillo will try to join the recent surge of Cuban players who have adapted well to the Major Leagues, a group that includes Red Sox outfielder Yoenis Cespedes, as well as White Sox slugger Jose Abreu, charismatic Dodgers outfielder Yasiel Puig and flame-throwing Reds closer Aroldis Chapman.
The contract Castillo signed includes the remainder of this season, and will expire following the 2020 season.
Red Sox manager John Farrell confirmed before Saturday's game that Castillo is likely to play some games for a Minor League affiliate before coming to the Majors at some point in September, when rosters are expanded.
Before playing in games, Castillo will report to the club's training facility in Fort Myers, Fla., where he can get his legs under him and get acclimated to life in professional baseball.
Castillo hit .319 (403-for-1,265) with 75 doubles, 11 triples, 51 home runs, 99 walks, 256 runs scored, and 76 stolen bases in 360 games over five seasons in Cuba's Major League, Serie Nacional. He played all five seasons for his hometown team, Ciego de Avila, and posted a career .383 on-base percentage, .516 slugging percentage, and .899 OPS while playing mostly center and right field.
After his initial attempt to defect from Cuba was unsuccessful, Castillo was suspended from game action in his homeland.
Given the current construction of the Red Sox's roster, Castillo's best fit would be in center field.
The Red Sox recently acquired corner outfielders Cespedes and Allen Craig in trades.
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.