The 6-foot-4 inch, 210-pounder, a first-round pick by Detroit in the 2002 NFL draft out of University of Oregon, will be the Saints' third-string signal-caller behind starter Drew Brees and backup Mark Brunell.
"Joey is a guy that we've kept our eye on," said Coach Sean Payton on the team's Web site.
"I've obviously had a few bumps along the way," Harrington told the Times-Picayune newspaper after he practiced with the Saints Friday. "Some mistakes I made (but) I think here, it gives me the chance to learn from somebody who's done it well in Drew."
Harrington, who also played for Miami and Atlanta, has thrown for 14,693 yards, with 79 touchdowns, 85 interceptions in six NFL seasons.
New Orleans had scheduled wide receiver Terrance Copper for the No. 3 quarterback slot, but the versatile five-year veteran has been slowed by a hamstring injury.