Only 41 percent of the US adults now say they have a favourable opinion of the Democratic presidential front-runner, with 51 percent saying that they hold an unfavourable view, the poll said.
The lowest favourability rating of 38 percent for Clinton came in 1992, Xinhua news agency reported.
However, Gallup said the sub-40 percent rating was caused by the public's lack of familiarity with her at that time rather than disapproval.
Clinton's deflated favorability rating comes at a time when the candidate is hit by controversy around her exclusive use of a private e-mail account and server while helming the US State Department.
The Federal Bureau of Investigation has launched an investigation into whether Clinton exchanged classified information via her private e-mail system, an assumption the Clinton camp has strongly denied.
Despite her dropping favourability rating, Gallup said Clinton remained well-liked among Democrats and independents who lean Democratic, with 74 percent having a favourable view of her.