The dog, named Deuce, was sold to an unknown party in the Stockton area Tuesday, according to police.
Anyone who recently bought a light brown and silver male Yorkshire terrier weighing about 7 pounds is asked to contact police at (510) 337-8340.
Deuce is licensed with the Alameda Animal Shelter and has an embedded ID chip. A reward is being offered for information leading to his safe return, police said.
The dog was stolen at about 2 p.m. Saturday when two men approached his elderly owner as she was walking near Encinal Avenue and Fountain Street, police said.
One man simulated a gun and demanded the woman's purse and dog. She refused to hand them over, so one suspect grabbed Deuce as the other wrestled the purse from her, police Lt. Sean Lynch said.
The robbers' vehicle, a blue 1995 Oldsmobile Cutlass Ciera, has been impounded, police said.