The objective of the study was to determine the polyphenolic composition of six blackberry cultivars, including the prevalently used Orkan and Gaj, and of four novel cultivars: Ruczaj, Polar, Brzezina, and Leśniczanka, as well as to determine the variability of main groups of polyphenols contained therein. Fruits of the above named cultivars constituted the research material; they were acquired from a Sadowniczy Zakład Doświadczalny Instytut Ogrodnictwa in Brzezna (Experimental Station for Fruit Growing, Institute of Horticulture in Brzezna, Poland). In the fruits, the basic groups of polyphenols were determined using a HPLC method: anthocyanins, ellagitannins, and flavan-3-ols. It was found that the contents of those polyphenols in blackberries were as follows: anthocyanins between 210 and 465 mg/100 g FW, ellagitannins between 53 and 320 mg/100 g FW, and flavan-3-ols between 121 – 282 mg/100 g FW; the total polyphenols were between 556 and 898 mg/100 g FW. The fruits of the Ruczaj cultivar were characterized by a significantly higher amount of anthocyanins (465 mg/100 g FW) and the Polar blackberries by a significantly lower amount of anthocyanins (210 mg/100 g FW.); however, the latter cultivar was characterized by a significantly higher amount of ellagitannins (319 mg/100 g FW). In the fruits of all the cultivars, cyanidin-3-O-glucoside was the main anthocyanin (79 – 91 % of total anthocyanins). In the blackberry fruits, the total contents of two ellagitannins: lambertianin C and sanguiin H-6, and their reciprocal percent contents were significantly different depending on the type of cultivar; thus, they could be considered to be a criterion for similarity. A high content of total polyphenols in the Ruczaj, Polar, and Gaj cultivars could be a further factor when popularizing the cultivation of those cultivars.