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This study aimed to evaluate the dosimetric benefits of magnetic resonance (MR)-guided radiotherapy (MRgRT) compared to computed tomography (CT)-guided RT for partial breast treatment. First, treatment plan quality was compared between MRgRT with step-and-shoot (S&S) IMRT and CT-guided RT with volumetric-modulated arc therapy (VMAT). Second, the dosimetric benefit of online adaptation in MRgRT was assessed by comparing the initial planned doses with daily MR-defined target geometry to those with planning CT-defined target geometry. Plan quality for MRgRT was comparable to, or slightly lower than, CT-guided RT. The V95% of the planning target volume (PTV) was lower by 1.72% and 1.94%, respectively, with a lower minimum dose and a higher maximum dose. For the OARs, the dosimetric parameters were generally similar, but for the skin, the Dmax, D0.5mL, and D1mL were slightly higher in MRgRT. Evaluation of the initial planned doses with the daily MR-based target definition resulted in a maximum deviation of 46% for the PTV V95%, demonstrating uncertainty in CT-based target definition. Although the MRgRT planning quality may be slightly compromised due to the S&S IMRT technique, the dosimetric benefits from online adaptation can be substantial when CT-based target definitions need to be modified based on daily MR imaging guidance.