Exactly how it gets rid of the abnormal build-up is not understood.
The small Nature Communications study hints at a way to tackle the disease even once its in full swing, dementia experts say.
But there is no proof the same method would work in people - many more years of animal trials are needed first.
Currently, there is no cure for Alzheimer's disease. Treatments can lessen the symptoms, but scientists are looking for ways to prevent, halt or reverse the disease.
As the dementia progresses, more plaques (clumps of abnormal proteins and chemicals) form in the brain and healthy brain cells die off.
Scientists reason that preventing or removing the plaques might help, and many drug candidates are in development.
Some drugs still being tested appear to stop the plaques from forming - but that is if it taken early enough, before the disease has advanced.
However, the South Korean researchers believe they may have found a molecule, called EPPS, that could work even if plaques have already formed.
They gave EPPS to mice (bred to have the Alzheimer's plaques) by spiking their drinking water for two weeks, and then monitored them over the next three months to see what effect it might have.
Compared with a control group of mice who received only normal water, the EPPS mice performed better on memory and learning problems (running through a maze).
The EPPS mice also had far fewer plaques in their brain at the end of the trial than they had had at the beginning. The same could not be said for the control group.
The Alzheimer's Society and Alzheimer's Research UK said it was important to remain cautious - animal study findings may never apply to humans.
Prof Tom Dening, an expert in dementia research at the University of Nottingham, said: "From a clinician's point of view, this research is of interest, but we still don't know if removing amyloid plaques is useful in humans.
"It may well be that the appearance of plaques is too far down the chain of molecular processes to be beneficial.
"We don't know if this animal work will lead to any useful agent that can be used for clinical trials."
A molecule can clear Alzheimer's plaques from the brains of mice and improve learning and memory, Korean scientists have found in early tests.