The two teams confirmed the move in a written statement Friday night and said Epstein would not meet with the media until Tuesday, the next off day for the World Series.
"Effective immediately, Theo Epstein has resigned from the Red Sox in order to become the new president of baseball operations for the Cubs," the joint statement said. "The clubs also have reached an agreement regarding a process by which appropriate compensation will be determined for the Red Sox and that issue will be resolved in the near term."
A Red Sox official told the Boston Herald the team is developing a list of candidates to replace Epstein. Whoever takes the job will hire a manager to replace Tony Francona and his coaching staff. Pitching coach Curt Young Friday became the third Boston coach to leave the club when he took the same job with the Oakland Athletics.