Buck has been Chelsea chairman since former owner Roman Abramovich bought the club in 2003.
The 76-year-old will however vacate his position and move into a senior advisory role as the Todd Boehly era begins to take shape.
Chelsea are now under new ownership following sanctions imposed on Abramovich for his links to Russia president Vladimir Putin, who is leading an invasion of Ukraine.
Boehly completed a £4.25b purchase of Chelsea last month after Abramovich put the club up for sale.
Under Buck's stewardship, Chelsea won 21 trophies, which included five Premier League titles, two UEFA Champions Leagues and two Europa Leagues.
Explaining his decision on the Blues’ official website, Buck said: “I am proud to have helped Chelsea realise great success on the pitch and make a positive impact in the community.
“Now is the right time to step down and let new ownership build on the strong foundations we have in place.
“The owners have a compelling vision for Chelsea’s future, and I look forward to helping them achieve it in this new role alongside our incredible staff, players, coaches and supporters.”
New owner Boehly added: “Bruce has led Chelsea to the highest levels of international and domestic football, while also developing one of the most active social responsibility projects in sport. We thank Bruce for his service and his commitment to the club.”
