D'Alessandro, the chairman of John Hancock Financial Services, will assume a limited partnership and becomes the 16th member of the group.
"Adding David to our group is good news for Boston and the Red Sox," Boston principal owner John Henry said. "His important, effective contributions and leadership spanning aspects of the sports and business worlds are well known. No one is more committed to our community than David."
"I am honored to be a new partner of the Boston Red Sox," D'Alessandro said. "They are a vibrant part of our community not only as a baseball team, but as a community citizen, too. I hope that I can, in some small way, contribute to their success."
D'Alessandro also serves with fellow Red Sox owner Tom Werner on the Commissioner's Initiative: Major League Baseball and the 21st Century. The committee comprises leaders from baseball, the media, entertainment, academia, marketing, publishing, and finance, is charged with analyzing all aspects of the game and how it is presented.
"David is among the most important people in sports," Werner said. "He has been the architect of John Hancock's sponsorship of Major League Baseball, the Olympic Games, and the Boston Marathon, and he has given us great support and advice ever since the acquisition."