"There was so much smoke. And there were two or three people who had a fire extinguisher. They were trying to help him," witness Everett Tyrrell told KOMO-TV.
"I was kind of worried about the guy. I guess his pants were affected by the phone or caught on fire," Tyrrell said. "He was pointing at the phone. I said, 'Is that your cellphone?' He said 'Yeah.' I asked, 'What type of cellphone? He said 'LG.'"
A store manager said the phone's flames had died down by the time employees arrived with a fire extinguisher. The manager said the man had managed to remove his pants fast enough to avoid being injured.
The manager said the phone left a scorch mark on the floor of the aisle.
The shopper was given a new pair of pants, the manager said.
It was unclear what caused the phone to burst into flames.
LG rival Samsung issued a recall last year for its Galaxy Note 7 phones, which were found to have a battery issue that caused some of the phones to explode and catch fire.