Catherine Greig entered her plea in a Boston courtroom. She has been charged with harboring and concealing a fugitive.
Prosecutors say Greig actively helped Bulger escape capture, but her attorney says she was in love with Bulger and wasn't aware of the extent of Bulger's alleged crimes when she fled with him in 1995.
The two were captured together in Santa Monica two months ago, more than 16 years after Bulger fled.
Grieg faces up to five years behind bars if convicted.
Bulger has pleaded not guilty to taking part in 19 murders while heading up Boston's notorious Winter Hill Gang.
The Associated Press contributed to this report.