Authorities say a woman walked out of Kohl's and headed to her car when the suspect pulled a semi-automatic weapon and asked for her purse at about 11 a.m. Sunday.
The suspect was described as about 80 years old, female and wearing a bright, floral scarf to cover her face.
"We believe there is a granny bandit out there," said Sgt. Billy Green of the Fontana Police Department.
Police say her getaway car is an older model dark sedan with tinted windows and a missing hubcap, possibly a Dodge Neon.
"Females tend to commit fewer violent crimes than males do, and when you compound it with the fact that it's an elderly, an extremely elderly woman, it becomes even more unusual," Green said.
The "granny bandit" struck again at a Target parking lot across town at about 9:30 p.m. Monday.
Police said a woman's purse was stolen, and the suspect and vehicle description were exactly the same.
"It's really sad that even the older people are doing that that. We're going to have to be afraid of everyone," said Maria Aguas of Fontana.
Authorities are now looking over surveillance tapes for clues.
"I couldn't imagine my grandma out there committing armed robberies," Green said.
Anyone with information about the "granny bandit" is urged to call the Fontana Police Department at (909) 350-7740.