A pulley device for a tensioner roller or winding roller of a transmission element, having a fixed inner element, a rotating outer element that is mounted to rotate coaxially about the fixed inner element, and an annular protective flange having a substantially radial portion bearing against a front surface of the fixed inner element. The flange includes an annular folded portion extending substantially axially from an outer edge of the flange and passing radially into the immediate vicinity of an axially protruding annular portion of the rotating outer element to form a narrow passage.