It told shoppers they can claim refunds for items they buy in both brick-and-mortar and online stores if these items are faulty.
"Retailers and consumers should be aware that discounted goods, like all other goods, are subject to complaint if they turn out to be faulty," the Office of Competition and Consumer Protection (UOKiK) said on its website.
Amid the coronavirus pandemic, many Poles have turned to online shopping to avoid contact with people.
When shopping online "remember to check the credibility of the retailer," the office warned.
"You should start with finding the registration details of the retailer, his address and contact details on the shop website," it said, adding that "typos, grammatical errors or a messy website design should be a red flag" for the buyer.
Black Friday and Cyber Monday are when both regular and online stores offer special deals to shoppers. The tradition originated in the United States but is increasingly popular in Poland as well.
The two days are widely seen as the beginning of the Christmas shopping season.
Black Friday follows Thanksgiving Day in the United States and this year falls on November 26.
Meanwhile, Cyber Monday is the Monday following Thanksgiving and sees major discounts offered by online retailers.
Source: IAR, uokik.gov.pl