Background: Leukemia is a cancer of the hematopoietic stem cells, which leads to an uncontrolled proliferation of leukocytes in blood. It is responsible for one of the most important cancer-associated deaths across the globe. Materials and Methods: In this study, we analyzed whether dieckol (DEK), a polyphenolic compound obtained from brown algae, can suppress cell proliferation via regulation of JAK/STAT3 signaling pathway in leukemia cell lines (Molt-4). Results: According to our results, DEK induced cytotoxicity, altered the cell morphology, caused nuclear damage, enhanced the formation of reactive oxygen species, decreased the production of mitochondrial membrane potential, reduced the levels of antioxidants (reduced glutathione, catalase, and superoxide dismutase), and augmented the level of thiobarbituric acid reactive substances in Molt-4 cell lines. Furthermore, STAT3 has been recognized as an important transcriptional mediator that controls cell proliferation. Thus, suppression of STAT3 transcription is a novel approach for the suppression of Molt-4 cell proliferation. In this study, DEK inhibited STAT3 translocation, thereby suppressing the increased expression of cyclin E1, PCNA, cyclin D1, and JAK1 in Molt-4 cell lines. Conclusion: In summary, DEK suppressed the cell proliferation of Molt-4 cells via inhibition of JAK/STAT3 signaling pathway.