Humala, a leftist former military officer who has governed from the center since taking office in July, campaigned on promises to fight corruption, attract investment and spread the wealth from the country's decade-long economic boom.
He is struggling, however, to govern as a moderate who can keep big business happy and help poor voters eager for change.
Humala's political honeymoon has also been cut short by the anti-mining protests and an illegal lobbying scandal involving one of his two vice presidents, Omar Chehade.
Despite falling 9 percentage points in December, the latest Ipsos survey showed the president's approval rating still outweighed his rejection rating of 42 percent.
Humala, who reshuffled his cabinet last week, had an approval rating of 65 percent in September, 62 percent in October and 56 percent in November.
The cabinet shake-up followed three weeks of protests in the northern Cajamarca region, where residents and local authorities are fighting a $4.8 billion gold and copper project led by U.S.-based Newmont Mining Corp.
While most people think the ministerial changes were necessary and approve of the new cabinet, president Humala hasn't emerged unscathed from the crisis, Ipsos Apoyo director Alfredo Toreros told El Comercio newspaper, which published the new poll.
In a bid to quell the protests in Cajamarca, Humala declared a state of emergency in the region, a measure that was supported by 73 percent of the poll's respondents.
The opinion survey polled 1,200 in cities nationwide from December 14-16. It has a margin of error of 2.8 percentage points.