The incident happened in front of the Sam's Club in the 1300 block of South Lone Hill Avenue around 1:45 p.m.
Glendora police said the gun was in 48-year-old Robert Duncan's fanny pack. They say his wife, 50-year-old Helen Duncan, was holding the pack, dropped it as they came out of the store, and the gun went off.
A bullet went through her husband's arm. He went to the hospital to be treated.
Police located the couple at the hospital and arrested both of them on suspicion of illegal possession of a concealed weapon and negligent discharge of a firearm. They were being held on $35,000 bail.
City News Service contributed to this report.