<p>Acidic soils are characterized by excess aluminum (Al) and low phosphorus (P) availability. However, the interactive effects of these elements within plants and the rhizosphere remain understudied. Red amaranth (<i>Amaranthus cruentus</i> L.), an Al-tolerant species, was precultured under sufficient (+ P) and deficient (‒P) phosphorus conditions, followed by treatment with Al for two weeks. A low ionic strength nutrient solution (pH 4.0) for avoiding Al-P precipitation, with an equal free Al³⁺ activity (60 µM) in the + P and −P mediums, was employed for plant treatment. Biomass and root elongation data indicated lower susceptibility of + P plants to Al³⁺ phytotoxicity compared with –P plants. Enhanced leaf accumulation of betacyanin and higher root exudation of phenolics, accompanied by less Al³⁺ binding to the root cell wall, were among the mechanisms for lower susceptibility of + P plants to Al³⁺ rhizotoxicity. However, our findings suggest the existence of some adaptive mechanisms acting under a combination of Al³⁺ toxicity with P deficiency. Phosphorus-deficient plants exhibited lower leaf Al accumulation, higher Al-induced H⁺ release, and acid phosphatase activity compared to + P plants. In addition, Al treatment enhanced the leaves’ total and free cytosolic inorganic P (Pi) levels and improved the plants ability to mobilize sparingly soluble phosphorus (Ca₃(PO₄)₃). Our data suggest that although sufficient P nutrition is necessary for full expression of adaptive mechanisms to Al³⁺ toxicity, modifications in the rhizosphere of –P plants mediated by Al³⁺ may significantly improve Pi availability and help maintain Pi homeostasis in plants growing on acidic soils.</p>