The former officer was dining at the restaurant, located on Foothill Boulevard and Woodward Avenue, at the time of the reported robbery.
The armed robbery took place at 7950 Foothill Blvd. just before 11 a.m. Friday.
There were about 20 customers in the restaurant during the robbery attempt.
"He struggled with the officer and at least one or two employees were involved in the struggle. The retired officer was actually fairly injured during the struggle, and wound up, in the end, discharging his weapon at the suspect," said LAPD Det. Pat Barron.
The suspect, a parolee, was in his mid-20s, according to police. He may have also robbed two other businesses in the area in recent days. Police say the suspect pistol-whipped one person during each of the previous robberies.
The suspect was not identified. Employees and the retired officer were hospitalized.