Dashboard footage shows the suspect, 64-year-old Otto Coleman, fighting with the officer and trying to take his gun and Taser.
Then, a woman showed up and started hitting Coleman, giving Seiter the upper hand just before his back-up arrived.
"I heard her go by and she was yelling call 911. When she was coming up, I saw her on the side and she commenced to help me out," Seiter said.
Police are not identifying the woman.
Seiter is very appreciative and has thanked her but says it is dangerous to do what she did.
Investigators say Coleman has a history of violence against police.