Lots of potentially good answers already, so I will try to keep my (initial) post short: before you can do anything to address the problem you must determine whether you have a leak from outside or have a condensation problem inside. ... I am assuming you have considered and discounted a plumbing leak.

If it's a leak from rainwater the problem will likely be year-round. If it's condensation it will likely be worse in winter .... unless you have air-conditioning which can, under some circumstances, cause condensation issues in the summer.

So solutions- if it's a leak, any plan must start with stopping the water entering the structure from the outside. All the interior wall treatments and dehumidification will be for nothing if the moisture is still entering the wall cavity, as it will just cause mold and rot.

If it's condensation, then running your AC will help, or a dehumidifier if the problem is only in one room. Also move furniture away from exterior walls to allow air to circulate. A fan (rather than a dehumidifier) might solve the problem if it's minor. In the winter an auxiliary heat source, such as an oil-filled radiator placed near the damp spot should also solve condensation dampness. .... When I was a child, I used to have a book case at the foot end of my bed, up against an exterior wall. We found mold had formed on the wall behind the bookcase, due to coldness and poor air circulation. Swapping the bookcase with a small radiator that already heated my room, entirely solved the problem. ... It also helped keep my feet warm! 😀