The 89-year-old was found dead in his cell at the maximum security facility shortly after being transferred there from a Florida jail.
Bulger was convicted in 2013 of being complicit in 11 mob killings. He was one of the FBI's most wanted fugitives for 16 years until his 2011 arrest in Santa Monica, California. The Oklahoma facility he was held in last week often serves as a temporary stop for inmates in transit.
The death of Bulger was announced by the Federal Bureau of Prisons on Tuesday, but a cause was not provided.
"Life-saving measures were initiated immediately by responding staff", the spokesperson said.
A jury convicted Bulger in 2013 on 31 criminal counts for his involvement in 11 murders across the country during his time as leader of the Boston-based Winter Hill Gang.
She was sentenced to eight years in prison for helping Bulger evade the law.
Bulger was arrested with his girlfriend, Catherine Greig.
Bulger, nicknamed "Whitey" for his bright platinum hair, grew up in a gritty South Boston housing project and became known as one of the most ruthless gangsters in Boston.
Bulger was later identified as the dead man, although officials released no details surrounding his sudden, violent demise. It's unclear if authorities have determined who played a role in the murder.
As they called him a coward, a rat, a punk and Satan, Bulger kept his head down and showed no emotion as he scribbled on a pad.