Gretzky, 18, is a 6-foot-4, 190-pound first baseman who has committed to play for San Diego State University. He reportedly received $375,000.
"The Cubs are thrilled to welcome Trevor Gretzky to the organization," general manager Jim Hendry said. "It was a pleasure working with the Gretzky family on this contract, and we are excited by Trevor's decision to begin his pro baseball career with the Cubs."
Gretzky, who hits left-handed and throws right-handed, hit .393 with 10 doubles and 27 RBIs last season for Oaks Christian High School in Westlake Village, Calif.
The Cubs still have some work to do before the signing deadline. They've yet to sign their first- and second-round selections, Javier Baez and Daniel Vogelbach. Baez, however, taken with the ninth overall pick, hinted on his Facebook page earlier this week that a signing could be close.
Teams have until 11:01 p.m. CT on Tuesday, Aug. 16, to come to terms with their draftees, meaning official notification has to be in the Commissioner's Office at that time. A team that does not sign its first- or second-round pick will receive a compensatory pick in the 2012 Draft. That selection will come at the same slot, plus one. In other words, if a team doesn't sign the No. 9 overall pick, it would receive the No. 10 pick -- technically 9A -- the following year. A team does not receive a 2012 pick if it does not come to terms with a selection made with a compensation pick this year.