Unemployment came in at 19 percent, followed by healthcare at 11 percent and various other issues at 10 percent, Gallup said.
The federal debt, education and immigration were mentioned by 5 to 8 percent of respondents.
Hispanics were among the most likely to name the economy and immigration as the most important problem, while blacks were most likely to mention unemployment and education, Gallup said.
Women under 50 years of age are more likely to mention healthcare and education, while seniors and liberals are more likely to name Congress as the main problem.
Although the economy is still the number one answer, fewer than 30 percent have mentioned the economy in Gallup's monthly measurement since December.
In most months between February 2008 and November 2012, at least 30 percent named the economy as the nation's top problem.
The poll was conducted July 10 through July 14. Telephone and cell phone interviews were conducted with 2,027 adults, and the margin of error was plus or minus 3 percentage points, Gallup said.