The robbery was reported at about 7:25 p.m. near the intersection of Calle Este and Calle Nogales.
The victim was walking home from the BART station and stopped at her mailbox to pick up the mail when a suspect approached and said, "Give me your purse or I'll shoot you," according to police.
The victim refused, but the suspect grabbed the purse and ran toward the BART station.
The victim's husband saw the suspect running away and chased him. The husband caught up to the suspect, who pulled out a handgun and threatened him.
The suspect escaped and was last seen near the north side of the BART station. No one was injured in the robbery, police said.
A police canine unit tracked the suspect to a 7-Eleven convenience store on Oak Road, but a search of the area came up empty.
The suspect is described as a black man who weighs about 200 pounds and is unshaven. He was wearing dark clothes and had a dark-colored handgun, according to police.
Anyone with information is encouraged to call the Walnut Creek Police Department at (925) 943-5844.