The Gallup-Healthways Well-Being Index, which questioned 356,599 Americans, including 18,322 unemployed adults, on their well being.
About 1 in 5 U.S. adults unemployed for a year or more said they currently have or are being treated for depression -- almost double the rate among those who have been unemployed for five weeks or fewer.
Gallup said 12.4 percent of unemployed Americans said they have or are being treated for depression compared with 5.6 percent of employed U.S. adults.
However, 18 percent of those unemployed for 27 weeks or longer reported depression.
Gallup poll was conducted Jan. 1 to Dec. 31, 2013, on the Gallup Daily tracking survey, with a random sample of 356,599 U.S. adults, age 18 and older.
The margin of error was less than 1 percentage point.