Ultrasound imaging velocimetry (UIV) is a maturing technique for measuring the dispersed phase in two-phase flows. It enables measurements of dense suspensions when optical methods fail. This study explores UIV’s applicability to measure the flow field in a swirling flow reactor (SFR) for solid–liquid mixing of dense suspensions. Despite UIV’s historical focus on unidirectional flows like arteries and axisymmetric pipes, this research demonstrates its adaptation to an inherently complex 3D flow field, i.e., a swirling sudden expansion flow in an SFR. Using high-speed plane-wave imaging and correlation averaging techniques, satisfactory velocity profiles are achieved while preserving sufficient temporal information. Firstly, the applicability of UIV in this specific setup is demonstrated by comparing UIV with stereoscopic particle image velocimetry measurements of a single-phase flow in the SFR, both indicating a Coandă jet flow (CoJF). Secondly, several bulk velocities and volume concentrations (up to 50 vol%) are measured with UIV for a suspension of water and 2.3-mm glass beads. A transducer is installed in two orientations and captures all three velocity components when combining the two datasets. A timestep optimization process is implemented to avoid the need for manual finetuning of the acquisition frequency. A time-domain spectral analysis on the dispersed phase velocity fields in the SFR reveals dominant frequencies between 1.21 and 2.42 Hz, similar to those found in single-phase flow. The general flow structure of the dispersed phase in suspension is very similar to the latter; however, the addition of particles confines the central recirculation zone (CRZ) to the center. Finally, the implementation of swirl to keep solid–liquid mixtures in suspension in the SFR is experimentally confirmed by this study. Quantitative UIV measurements confirm favorable flow structures for mixing, specifically a CoJF that avoids sedimentation. The concentration of solids in an SFR can even be increased up to 50 vol% while still maintaining a uniform suspension.