The primary aim of this study is to address the challenges in submillimeter diffusion magnetic resonance imaging (dMRI), such as prolonged acquisition time, low signal-to-noise ratio (SNR), and signal attenuation at slab boundary. We introduce a novel 3D Fourier encoding mechanism, PRISM (Partition-encoded Simultaneous Multislab), and a new concept termed "pseudo slab." The PRISM method allows simultaneous inter-slab and intra-slab Fourier encoding solely using the slice gradient, eliminating the need for RF encoding. The pseudo slab concept not only minimizes inter-slab signal leakage and Gibbs truncation artifacts, but also enables phase scheduling onto intra-slab slices, thus eliminating the need for a phase navigator and time-varying gradient such as variable-rate selective excitation (VERSE). Integrating the pseudo slab with PRISM, the resulting pseudo PRISM (pPRISM) technique achieved rapid acquisition of dMRI with 0.86-mm isotropic resolution and an effective TR of 12 s (TR of 2.4 s per shot). Compared to Generalized Slice Dithered Enhanced Resolution with Simultaneous Multislice (gSlider-SMS), the shortened acquisition time improved the SNR efficiency without aggravating the signal attenuation at slab boundaries. The robustness of pPRISM against field inhomogeneity was also supported by Bloch simulation and empirical data. Furthermore, dMRI was successfully achieved with a 0.76-mm isotropic resolution, an effective TR of 15 s, and b-values of up to 2500 s/mm. The ultrahigh-resolution results of the proposed pPRISM method demonstrated the anticipated dark bands of fractional anisotropy (FA) at gray-white matter boundaries and yielded more plausible tractography results. Our pPRISM framework paves the way for acquiring ultrahigh-resolution dMRI in clinically feasible times, advancing microstructural research.