Prosecutors say Catherine Greig, 60, will plead guilty to helping Bulger evade capture for more than 16 years after he fled Boston.
Bulger, 82, headed the notorious Winter Hill Gang and was a top-echelon FBI informant who ratted out the rival New England Mafia. His former FBI handler, John Connolly Jr., was convicted for warning him that he was about to be indicted, prompting him to flee Boston in late 1994.
Bulger is charged with participating in 19 murders. He has pleaded not guilty. Greig, who was arrested with Bulger in Santa Monica last June, is charged with conspiracy to harbor a fugitive and identity fraud.
Families of Bulger's victims say Greig could face as few as 32 months in prison.
The Associated Press contributed to this story