The rating is lower than the 64 percent she averaged while serving in the cabinet of President Barack Obama, Gallup reported.
The last time a majority of respondents rated her as unfavorable, she was running against Obama in the Democratic presidential primaries.
Her ratings have risen and fallen since the first survey was conducted in 1992. The rating peaked at 67 percent in late 1998, just after her husband was impeached.
Only 46 percent of those asked rate Vice President Joe Biden as favorable. The rating has been in the 40 percent range for the entirety of his time as vice president, Gallup said.
Clinton and Biden may battle for the Democratic nomination in the 2016 presidential election, but neither has declared their candidacy.
The poll was conducted between Feb. 6 and Feb. 9 by telephone interviews. The margin of error was 4 percentage points.