The other top winners: Kate Winslet, best actress for the Holocaust-themed drama "The Reader"; Sean Penn, best actor for the title role of "Milk"; Heath Ledger, supporting actor for "The Dark Knight"; and Penelope Cruz, supporting actress for "Vicky Cristina Barcelona."
A story of hope amid squalor in Mumbai, India, "Slumdog Millionaire" came in with 10 nominations, its eight wins including adapted screenplay, cinematography, editing and both music Oscars (score and song).
"Just to say to Mumbai, all of you who helped us make the film and all of those of you who didn't, thank you very much. You dwarf even this guy," Boyle said, holding up his directing Oscar.
The filmmakers accepted the best-picture trophy surrounded by both the adult professional actors who appeared among the cast of relative unknowns and some of the children the British director cast from the slums of Mumbai.
The film follows the travails and triumphs of Jamal, an orphan who artfully dodges a criminal gang that mutilates children to make them more pitiable beggars. Jamal witnesses his mother's violent death, endures police torture and struggles with betrayal by his brother, while single-mindedly hoping to reunite with the lost love of his childhood.
Fate rewards Jamal, whose story unfolds through flashbacks as he recalls how he came to know the answers that made him a champion on India's version of the TV game show "Who Wants to Be a Millionaire?"
"Slumdog Millionaire" also received the biggest bounce at the box office of the best-picture nominees.
The rags-to-riches tale brought in an extra $54 million domestically since its nomination Jan. 22 - more than doubling its gross since its November premiere, to $98 million to date.
By David Germain