The actor apparently died of a drug overdose, said two law enforcement officials, who spoke to The Associated Press on condition of anonymity because they were not authorized to discuss the case. Envelopes containing what was believed to be heroin were found with him, they said.
Hoffman - with his doughy, everyman physique, his often-disheveled look and his limp, receding blond hair - was a character actor of such range and lack of vanity that he could seemingly handle roles of any size, on the stage and in movies that played in art houses or multiplexes.
He could play comic or dramatic, loathsome or sympathetic, trembling or diabolical, dissipated or tightly controlled, slovenly or fastidious.
The stage-trained actor's rumpled naturalism brought him four Academy Award nominations - for "Capote," ''The Master," ''Doubt" and "Charlie Wilson's War" - and three Tony nominations for his work on Broadway, including his portrayal of the beaten and weary Willy Loman in "Death of a Salesman."
Hoffman spoke candidly over the years about his struggles with drug addiction. After 23 years sober, he admitted in interviews last year to falling off the wagon and developing a heroin problem that led to a stint in rehab.
"No words for this. He was too great and we're too shattered," said Mike Nichols, who directed Hoffman in "Charlie Wilson's War" and "Death of a Salesman."
The law enforcement officials said Hoffman's body was discovered in a bathroom in his Greenwich Village apartment by his assistant and a friend who made the 911 call.
For much of the day, a police crime-scene van was parked out front, and technicians carrying brown paper bags went in and out. Police kept a growing crowd of onlookers back. A single red daisy had been placed in front of the lobby door.
On Sunday night, a black body bag was carried out on a stretcher, loaded into the back of a medical examiner's van and driven away.
Hoffman's family called the news "tragic and sudden." Hoffman is survived by his partner of 15 years, Mimi O'Donnell, and their three children.
"We are devastated by the loss of our beloved Phil and appreciate the outpouring of love and support we have received from everyone," the family said in a statement.
Tributes poured in from Hollywood figures.
"Damn, We Lost Another Great Artist," Spike Lee, who directed him in "25th Hour," said on Twitter.
Kevin Costner said in an AP interview: "Philip was a very important actor and really takes his place among the real great actors. It's a shame. Who knows what he would have been able to do? But we're left with the legacy of the work he's done and it all speaks for itself."
Hoffman was as productive as he was acclaimed, often appearing in at least two or three films a year while managing a busy life in the theater.
He was a spoiled prep school student in one of his earliest movies, "Scent of a Woman," in 1992. One of his breakthrough roles came as a gay member of a porno film crew in "Boogie Nights," one of several movies directed by Paul Thomas Anderson that he would eventually appear in.
He played comic, slightly off-kilter characters in movies like "Along Came Polly," ''The Big Lebowski" and "Almost Famous." And in "Moneyball," he was Art Howe, the grumpy manager of the Oakland A's who resisted new thinking about baseball talent.
He was nominated for the 2013 Academy Award for best supporting actor for his role in "The Master" as the charismatic, controlling leader of a religious movement. The film, inspired in part by the life of Scientology founder L. Ron Hubbard, reunited the actor with Anderson.
He also received a 2008 best-supporting nomination as a CIA officer in "Charlie Wilson's War," and another such nomination in 2009 for "Doubt," in which he played a priest who comes under suspicion because of his relationship with a boy.
Many younger moviegoers know him as Plutarch Heavensbee in "The Hunger Games: Catching Fire," and at the time of his death he was reprising that role in the two-part sequel, "The Hunger Games: Mockingjay," for which his work was mostly completed.
Lionsgate, which distributes the adaptations of Suzanne Collins' multimillion-selling novels, called his death a tragedy and praised him as a "singular talent." The last two "Hunger Games" movies are scheduled for release in November 2014 and November 2015.
Just weeks ago, Showtime announced Hoffman would star in "Happyish," a new comedy series about a middle-aged man's pursuit of happiness.
Born in 1967 in Fairport, N.Y., Hoffman was an athletic boy, but a neck injury suffered while wrestling ended any hopes of a career in sports. He soon became interested in acting, mesmerized at 12 by a local production of Arthur Miller's "All My Sons."
He studied theater as a teenager with the New York State Summer School of the Arts and the Circle in the Square Theatre. He then majored in drama at New York University.
In his Oscar acceptance speech in 2006 for "Capote," he thanked his mother for raising him and his three siblings alone, and for taking him to his first play. Hoffman's parents divorced when he was 9.
On Broadway, he took on ambitious parts like Willy Loman in "Death of a Salesman," Jamie in "Long Day's Journey Into Night" (his parents were played by Brian Dennehy and Vanessa Redgrave) and both leads in "True West." All three performances were Tony nominated.
His 2012 work in "Death of a Salesman" was praised as "heartbreaking" by AP theater critic Mark Kennedy.
"Hoffman is only 44, but he nevertheless sags in his brokenness like a man closer to retirement age, lugging about his sample cases filled with his self-denial and disillusionment," Kennedy wrote. "His fraying connection to reality is pronounced in this production, with Hoffman quick to anger and a hard edge emerging from his babbling."
Last year, he crossed to the other side of the footlights to direct Bob Glaudini's "A Family for All Occasions" for the Labyrinth Theatre Company, where he formerly served as co-artistic director. Hoffman also directed "Jesus Hopped the A Train" and "Our Lady of 121st Street" for the company and received Drama Desk Award nominations for both productions.
His other notable off-Broadway roles include Iago in Shakespeare's "Othello" in 2009, Konstantin in "The Seagull," directed by Nichols in 2001, and a Drama Desk-nominated part in Glaudini's "Jack Goes Boating" in 2007.
Two films starring Hoffman premiered last month at the Sundance Film Festival: the espionage thriller "A Most Wanted Man" and "God's Pocket."
Associated Press writer Deepti Hajela, AP Drama Writer Mark Kennedy and AP National Writer Hillel Italie contributed to this report from New York.