A simple one-pot reaction of maleic anhydride with iminodi(methylphosphonic acid) in presence of N,N’-methylenebisacrylamide allows producing a micron-sized multifunctional sorbent (herein called MPOH-MBA, bearing both carboxylate, phosphonate groups, and tertiary amine groups). These multifunctional groups bring high reactivity for rare earth elements (REEs, hard acids) (soft bases O-bearing ligands, consistently with Pearson’s principle) with modulation of positively-charged surface (amine groups for interaction with scandium sulfate species). Effectively, MPOH-MBA shows remarkable sorption capacity for Sc(III) at optimum initial pH (i.e., pH0: 4): qeq,exp reaches up to 5.34 mmol Sc g−1, at room temperature. Langmuir equation fits well experimental profiles (affinity coefficient: 0.677 L mmol−1). Scandium sorption is endothermic: sorption capacity increases up to 6.50 mmol Sc g−1 (at T: 50 °C), while the affinity coefficient increases up to 1.54 L mmol−1. Fast kinetics (equilibrium reached in 20–30 min) are favored by the micron-size of the sorbent (and its good textural properties: 51 m2 g−1 for specific surface area and pore width close to 250 Å). Kinetic profiles are fitted by the pseudo-first order rate equation. The sorbent can be easily regenerated using 0.3 M HCl solution; the loss in sorption is less than 1.5% at the fifth cycle. Main reactive groups (identified by Fourier-transform infrared spectroscopy) are carboxylate and phosphonate groups. In presence of equimolar concentrations of competitor ions: sorption is governed by preference for trivalent metal ions over divalent cations. Among trivalent metal ions rare earth elements (REEs) are preferentially bound (against Al(III) and Fe(III)); in addition, MPOH-MBA sorbs Sc(III) with higher affinity than Nd(III) and Ce(III). The sorbent shows a relative selectivity for scandium and REEs over metal ions present in huge excess (100–500 excess), such as Fe(III) and Al(III)), in red mud acidic leachate.