The Associated Bank's surveillance video shows the man at a counter when the masked robber carrying a gun rushes to a teller at the bank in St. Francis at about 1 p.m. Wednesday.
Within five seconds, the man approaches the robber from behind. A second later he puts his arm around the robber's neck and jumps on his back.
The robber then tries to run with the man on his back.
Police Chief Brian Kaebisch says the man eventually released the robber before the robber ran to a car. No one was hurt.
Kaebisch says he appreciates the effort, but police don't want people intervening in robberies. He would rather bystanders be good witnesses.