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Purpose To quantitatively evaluate a superresolution technique for 3D, one‐millimeter isotropic diffusion‐weighted imaging (DWI) of the whole breasts. Methods Isotropic 3D DWI datasets are obtained using a combination of (i) a readout‐segmented diffusion‐weighted‐echo‐planar imaging (DW‐EPI) sequence (rs‐EPI), providing high in‐plane resolution, and (ii) a superresolution (SR) strategy, which consists of acquiring 3 datasets with thick slices (3 mm) and 1‐mm shifts in the slice direction, and combining them into a 1 × 1 × 1‐mm3 dataset using a dedicated reconstruction. Two SR reconstruction schemes were investigated, based on different regularization schemes: conventional Tikhonov or Beltrami (an edge‐preserving constraint). The proposed SR strategy was compared to native 1 × 1 × 1‐mm3 acquisitions (i.e. with 1‐mm slice thickness) in 8 healthy subjects, in terms of signal‐to‐noise ratio (SNR) efficiency, using a theoretical framework, Monte Carlo simulations and region‐of‐interest (ROI) measurements, and image sharpness metrics. Apparent diffusion coefficient (ADC) values in normal breast tissue were also compared. Results The SR images resulted in an SNR gain above 3 compared to native 1 × 1 × 1‐mm3 using the same acquisition duration (acquisition gain 3 and reconstruction gain >1). Beltrami‐SR provided the best results in terms of SNR and image sharpness. The ADC values in normal breast measured from Beltrami‐SR were preserved compared to low‐resolution images (1.91 versus 1.97 ×10–3 mm2/s, P = .1). Conclusion A combination of rs‐EPI and SR allows 3D, 1‐mm isotropic breast DWI data to be obtained with better SNR than a native 1‐mm isotropic acquisition. The proposed DWI protocol might be of interest for breast cancer monitoring/screening without injection.