To provide a screw propeller and a propulsion unit capable of surely preventing or suppressing lowering of propulsive force obtained by rotation of a propeller for a vessel when the propeller for the vessel rotates in the water.

The propeller 4 for the vessel includes a boss 41 and a plurality of blades 5 arranged along a peripheral direction in an outer peripheral part of the boss 41. In each blade 5 of the propeller 4 for the vessel, each rear edge 54 located on the rotating direction rear side when the propeller 4 for the vessel rotates forms bent or curved waveforms at a plurality of portions. Each blade 5 includes each of projecting parts 57a to 57d which is projected and formed on a back surface 52 and extends toward a front edge 53 on an opposite side of each rear edge 54 from each crest 541 of the waveforms.