The robber, who earned his nickname by wearing a distinctive cloth over his face, is believed to have hit banks in Bellevue, Woodinville, Shoreline, Seattle, Kirkland and Bothell.
Three of the robberies were in October, and the latest was in Seattle's Northgate area on December 4, according to the FBI.
Authorities believe the robber is a white man who stands between 5'9" and 6-feet tall and has a thin build. During the robberies, he covers his face and wears the hood of his black or orange sweatshirt over his head. He also wears clear, latex gloves.
Wells Fargo is offering a reward of up to $5,000 for information leading to the identification and arrest of the bank robber.
Anyone with information about the robber is asked to call Crime Stoppers at 1-800-222-TIPS (8477).