According to Deadline, the movie knocked out 'Get Out' and 'Phantom Thread' to secure the position.
'Lady Bird' also took back the trophy in the best director, screenplay and supporting actress categories.
Here's the complete winners list:
Best picture: ?Lady Bird
Best actor: Daniel Kaluuya for Get Out
Best actress: ?Sally Hawkins for The Shape of Water and Maudie
Best supporting actress: ?Laurie Metcalf for Lady Bird
Best supporting actor: ?Willem Dafoe for The Florida Project
Best cinematography: Roger Deakins for Blade Runner 2049
Best foreign-language film: Graduation
Best non-fiction film: Faces Places
Special citation for a film awaiting U.S. distribution: Agnieszka Holland's Spoor
Best experimental film: Ben Russell's Good Luck
Film heritage award: "One Way or Another: Black Women's Cinema 1970-1991," BAMcinematek
Film heritage award: Dan Talbot for his pioneering work as an exhibitor and distributor in bringing worldwide cinema to the United States. (ANI)