In a statement, the intelligence service said it has enough evidence to conclude that North Korea was behind the punishing breach, which resulted in the disclosure of tens of thousands of leaked emails and other materials.
“North Korea's actions were intended to inflict significant harm on a US business and suppress the right of American citizens to express themselves. Such acts of intimidation fall outside the bounds of acceptable state behavior," the statement said.
The FBI's case cited, among other factors, technical similarities between the Sony break-in and past "malicious cyber activity" linked directly to North Korea.
President Barack Obama’s administration officials had previously declined to openly blame North Korea but said they were weighing various options for a response.
The statement on Friday did not reveal what options were being considered. President Obama is expected to face questions about the Sony hack at a year-end news conference with reporters later on Friday.
The break-in escalated to terrorist threats that prompted Sony to cancel the Christmas release of the movie "The Interview."
The film stars Seth Rogan and James Franco is a comedy romp in which two journalists are contacted by North Korea to take part in a staged Q&A, and are then hired by US agents to kill Kim Jong Un.
A group of hackers known as Guardians of Peace (GOP) had earlier claimed responsibility for the attack.
TIME magazine writes: "Early reports suggested North Korea was behind the GOP, and there's been some evidence of that. But North Korea has denied responsibility for the hack, and it's equally possible the assailants planted clues leading to North Korea as a distraction."
It's not immediately clear what action, if any, the US government will take. Bringing the shadowy hackers to justice appears a distant prospect. A cyber-retaliation against North Korea would risk a dangerous escalation. And North Korea is already targeted by a raft of sanctions over its nuclear weapons program.
The FBI did not indicate whether it has identified any individual hackers who might be culpable.
In May, the Justice Department indicted five Chinese military officers accused of vast cyberespionage against American corporate interests, but none of those defendants has yet to set foot in an American courtroom.