The picture, which appears to have been taken from behind the counter, was posted to the social news site Reddit.
The unidentified employee is shown eating the soft-serve ice cream treat with the caption "I was going to buy a frosty from Wendy's until I saw the employee do this."
When the photo went viral Wednesday, the fast food chain responded.
In an e-mail to the Milwaukee Journal Sentinel, Wendy's spokesman Bob Bertini says "If true, this is totally inexcusable. We are investigating and will take action."
The incident follows a similar one last week when a Taco Bell employee was photographed licking a stack of taco shells.
That employee has since been fired.