A 14-year-old boy has been charged with fatally stabbing Tessa Majors, a Barnard College student, during a park robbery near her school in December.

The Manhattan district attorney, Cyrus R. Vance Jr., said the boy, Rashaun Weaver, would be tried as an adult on two counts of second-degree murder and several counts of robbery.

Investigators had long believed that the teenager wielded the knife that killed Ms. Majors after she bit his hand during a violent struggle with three assailants. Ms. Majors, 18, was found severely wounded on steps outside Morningside Park the night of Dec. 11.

The arrest brought a sense of relief to the authorities, who were under pressure to solve the first high-profile case under the new police commissioner, Dermot F. Shea.