Background: In this study, intensity-modulated radiation therapy plans were made with and without couch rotation in patients with right unilateral breast cancer, and a dosimetry analysis was carried out to compare the radiation doses received by target and normal tissues. Materials and Methods: The radiotherapy planning tomography sets of 10 patients who underwent right unilateral mastectomies were retrospectively selected. Target volumes and normal at-risk organs were recontoured, two radiotherapy plans were created for each patient, and these plans were compared by dosimetry analyses. Results: Doses in the target volume (D2%, D98, D50, HI, VRI, and T-PTV-V95) were similar between the plans. In terms of organs at-risk doses, the maximum doses in the contralateral breast were similar between the plans, while the differences in all other organs at-risk dose parameters between the plans were statistically significant. All dosimetry parameters of the heart were significantly lower in the plans with couch rotation. Ipsilateral lung doses were higher in the plans with couch rotation. Contralateral lung and mean breast doses were significantly lower in the plans with couch rotation. Conclusion: In this study, organs at-risk doses were reduced, especially for the heart and the contralateral breast, in patients who were subjected to postmastectomy radiotherapy with right thoracic wall and regional nodal irradiation without compromising radiotherapy dose coverage for the target volumes by rotating the treatment couch by 270°.