Jeptoo pulled away from a pack of rivals that included US runner Shalane Flanagan, who had led the women's group for the first 20 miles of the 26.2-mile race, setting a blistering pace at the first running of the Boston Marathon since last year's fatal bombing attack.
The previous women's record for the course was a time of 2:20:43 set in 2002 by Margaret Okayno of Kenya.
Jeptoo finished with an unofficial time of two hours, 18 minutes and 57 seconds. Buzunesh Deba of Ethiopia was second and compatriot Mare Dibaba third. (Reporting by Scott Malone; Editing by Justin Palmer)