The role of sulfur (S) in determining both seed yield and seed quality makes it important to understand the trade-off between these two traits with respect to S utilization. We started to address this question in pea by investigating the physiological relevance of vacuolar sulfate remobilization during the reproductive period. We targeted the only vacuolar sulfate transporter SULTR4 present in pea: PsSULTR4. Its simulated 3D structure appeared to be conserved with that experimentally determined for the Arabidopsis transporter AtSULTR4;1 (Wang et al. 2021), which mediates sulfate efflux from the vacuole (Kataoka et al. 2004). Five mutant and wild-type lines were phenotyped under S-sufficient and S-deficient conditions. One mutation in the cytosolic domain (E568K) reduced seed yield under S deficiency to a similar extent as a nonsense mutation, highlighting this residue as essential for sulfate transport activity. Interestingly, mature seeds of these two mutants developed under S sufficiency showed changes in seed protein composition characteristic of S-deficiency, with less S-rich storage proteins. This was associated with reduced sulfate utilization within the seed. Gene expression studies and sulfate measurements in developing seed tissues revealed the importance of vacuolar sulfate remobilization in the embryo-surrounding tissues for the synthesis of S-rich seed proteins. These data shed light on the critical role of PsSULTR4 in balancing whole plant sulfate homeostasis to maintain seed yield and seed quality under variable S supply.