Investigators said the suspect entered the Old Knickerbocker Hollywood Hotel, which has been converted into a retirement home, located near Hollywood Boulevard and Ivar Avenue at about 4 a.m. on Saturday.
The man got into the building by dialing multiple apartments on the call box until someone buzzed him inside, detectives said.
Walking past the security office, the man took the elevator to the upper floors of the complex and knocked on the 80-year-old woman's door.
Police said when she opened the door, the suspect pushed his way in, severely beat her and tried to sexually assault the woman.
"She fought back, and after fighting back, she beat back the suspect as much as possible and then he became very, very aggressive and significantly beat her up," Deputy Chief Matt Blake with the Los Angeles Police Department said.
Detectives said the man then ran from the scene. The woman, left bloodied from the attack, was able to make it to a neighbors door, who then called police.
Paramedics arrived and rushed the woman to the hospital. Police said the woman was in critical condition.
Authorities said the suspect was described as a black male, between the ages of 25 and 30, with black hair and brown eyes, standing about 6-foot tall and weighing 220 pounds.
Police said suspect was last seen wearing a black hooded sweatshirt with "Penn State" written on the front in white letters, black baggie jeans and black shoes.
Anyone with information that could aid detectives in their investigation was urged to call Crime Stoppers at 1-800-222-8477. Tipsters can remain anonymous.