A hydraulically powered fishing tool is provided for retrieving another tool or tubular stuck in a well. A tool housing is supported in a well on a work string, and the housing encloses a plurality of pistons (16, 28, 34) each movable in response to pressurized fluid transmitted through the work string. An anchor (52) axially fixes the position of the tool in the well, and a tool mandrel (40) is axially movable relative to tool housing when the anchor is set. A fishing device (82) engages the another tool or tubular, so that axial movement of the mandrel in response to the plurality of pistons dislodges the stuck tool or tubular.