Humala, who has been in office since July 2011, saw his popularity slip to 25 percent from 27 percent in November, stung by a police corruption scandal, pollster Ipsos said.
His approval rating has plummeted 29 percentage points since March and about 69 percent of Peruvians now disapprove of the former military officer.
That's a far cry from the roughly 65 percent approval ratings he enjoyed near the start of his term.
While Humala has put in place a variety of social programs, he has been criticized for not doing enough to ensure the poor benefit from an ongoing metals boom in the Andean country.
The Ipsos Peru poll of 1,663 people has a margin of error of 2.4 percentage points.