The robbery occurred around 8:55 p.m. at an apartment on Spencer Street off of 16th Street between Guerrero and Dolores streets.
According to police, the 79-year-old man let the suspect, a woman between 30 and 35, come into his apartment.
She walked into the kitchen, grabbed a box cutter and continued into the victim's bedroom.
When the victim confronted her, police said she threatened him with the weapon. She then took cash from the man and left the apartment without injuring him.
The woman had not been arrested as of this morning, according to police.