The annual "Free November" initiative is being held for the 11th time, public broadcaster Polish Radio’s IAR news agency has reported.
Tourists can check out attractions such as the Royal Castle in Warsaw, the historic Wawel complex in the southern city of Kraków, and the Castle of the Teutonic Order in the northern town of Malbork.
Other sites available to the public for free this month include Warsaw's Wilanów Palace and the Royal Łazienki Museum in the Polish capital.
"This unique campaign aims to expand access to our beautiful museums and royal residences," said Polish Deputy Prime Minister and Culture Minister Piotr Gliński during a press conference in Warsaw last week.
"It is taking place for the 11th time in response to requests from the public for better and easier access to museums," he added.