The dog's owner, Alfonso Galindo, said the 7-year-old poodle mix, Pupa, did nothing to provoke such a reaction.
Galindo told Inside Edition he decided to check the surveillance footage after he returned to his Otay Mesa, California, home and noticed Pupa rolling on the ground, seemingly trying to wipe something away from her eyes.
"It's heartbreaking to see, it really is, to see our dog suffering in that manner," said Galindo.
The video showed Pupa waiting safely behind a gate as the postal worker approaches, carrying mail. From under the pile of letters, Galindo said the worker pulls out the apparent pepper spray and hits Pupa with it.
"You can see his finger on the trigger," said Galindo as he showed Inside Edition the video. "He decides to walk right in front of the gate and nonchalantly spray the dog."
Galindo said the spraying happened not once, but 11 times.
The dog is kept 3 feet from the mail box, according to Galindo. "My gate is over here, where my dog is obviously behind," he said.
The spray affected not only Pupa, Galindo said, but his children as well, who were exposed as they played with their beloved dog. "They've been sick with respiratory issues," he said, adding that his family has visited the emergency room "countless" times.
Galindo said he's never received any complaints about Pupa.
In a statement, the USPS said: "On behalf of the United States Postal Service, we want to apologize to the Galindo family and Pupa. We do not condone our employees behaving in a manner which is not professional and courteous.The appropriate personnel and corrective action will be taken as well as training given to all local letter carriers."
The worker has not been identified.