Previous studies on the interaction between environmental inorganic phosphate (Pi) and salinity stress using soybean cultivars sensitive to high external Pi had two limitations: (1) the phenotype was dominated by overaccumulation of phosphorus (P); and (2) no detailed analysis was performed for sodium ion uptake. In this study, we focused on the effects of high external Pi on the sodium ion uptake in 'Pi-tolerant' soybean cultivars. The P accumulation in Pi-tolerant soybean Union was much lower [9.0 mg g(-)(1) dry weight (DW); contrasting to 38-76 mg g(-)(1) DW in the 'Pi-sensitive' soybean cultivars]. At in planta level, high level of external Pi significantly (P < 0.001) increased net sodium ion uptake and aggravated salinity stress symptoms. The effects of high external Pi diminished when de-rooted plants were used, suggesting that root is the primary organ interacting with Pi in the growth medium. Two-cell models, including soybean suspension cells and the tobacco Bright Yellow-2 cell line, were also employed to study the effects of high external Pi at the cellular level. Consistent to in planta results, high external Pi uplifted cellular sodium ion uptake and reduced cell viability under salinity stress. Gene expression analyses further showed that HPi (2 mM Pi supplements; excessive level of Pi) could reduce the fold of induction of GmSOS1 and GmCNGC under salinity stress, suggesting that they may be possible molecular targets involved in the interaction between high external Pi and Na(+) uptake.