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Obesity is associated with breast cancer aggressiveness and drug resistance. Although the underlying mechanisms are unknown, recent studies indicated that exosomes have a principal contributory role in obesity‐associated metabolic complications. Hence, we investigated whether obesity can mediate breast cancer progression and resistance to tamoxifen by plasma‐derived‐exosomes from obese women or not. Plasma exosomes isolated from five normal‐weight (N‐Exo) and five obese women (O‐Exo) were characterized for size, zeta potential, and CD63 expression. After the treatment of MCF‐7 cells with N‐Exo and O‐Exo, cell proliferation, migration, invasion as well as levels of MMP‐9 and MMP‐2 were evaluated by the 3‐(4,5‐Dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) assay, wound healing, transwell, and zymography methods, respectively. For evaluating resistance to tamoxifen, the cell viability, apoptosis, and the p53 protein were evaluated using the MTT assay, flow cytometry, and western blot methods, respectively. Cell proliferation, migration, and invasion were significantly increased in the cells treated with O‐Exo than untreated cells (p =.001, p =.018, p =.034, respectively). Levels of MMP‐2 and MMP‐9 were remarkably increased in the cells treated with O‐Exo in comparison with ones treated with N‐Exo (p =.040, p =.043, respectively). As for resistance to tamoxifen, O‐Exo had significantly the greater anti‐apoptotic effects in comparison with the N‐Exo group (p =.013). Besides, p53 levels were significantly decreased in the cells treated with O‐Exo than ones treated with N‐Exo (p =.045). The cell viability was significantly more in cells treated with O‐Exo in comparison with the cells only treated with tamoxifen (p =.040). Our findings demonstrated that circulating exosomes derived from obese women could lead to tumorigenesis and tamoxifen resistance in breast cancer cells. However, more studies are needed to establish this notion. [ABSTRACT FROM AUTHOR]