Gallup said Wednesday the low for October matches that of December 2010 and August 2011.
Low ratings transcend party lines, with 14 percent of both Democrats and Republicans saying they approve of Congress' performance, and 13 percent of independents, Gallup said.
The ratings have been consistently low in recent months, below 20 percent each month since June.
Older Americans gave Congress the lowest marks. Of those 55 and older, only 8 percent said they approved of Congress' performance, continuing a trend of single-digit ratings dating to July for that age group.
Among those 35 to 54, 12 percent said they approved of Congress' performance, and among those 18 to 34, 21 percent.
Differences in approval by age are more pronounced now than they historically have been, which could influence congressional races next year, as older Americans are typically more likely to vote, Gallup said.
Gallup said if the low ratings pattern continues, Congress will finish 2011 with its lowest performance rating for any year since the organization began measuring congressional approval in 1974. Existing lows are 19 percent, in 1992, based on a single measure, and 19 percent for 1979 and 2008, both based on monthly ratings.
The latest poll results are based on telephone interviews conducted Oct. 6-9 with a random sample of 1,005 adults. The sampling error is 4 percentage points.