The Reds signed the 32-year-old righty to a minor league contract. This is the fourth straight spring that the oft-injured pitcher has tried to make it back to the majors.
Prior pitched for Boston's Triple-A affiliate last season, making 19 relief appearances with a 3.96 ERA.
The move reunites Prior and Reds manager Dusty Baker. They were together with the Chicago Cubs several years ago, and Prior was an All-Star in 2003, when the team came close to reaching the World Series.
Baker said Friday that he's kept up with Prior since their days in Chicago.