Carbon spheres (CSs) have become a recent focus of attention in environmental remediation techniques. In this study, imine-functionalized CSs were synthesized from plant extract (peCSs) for the first time and effectively used in U(VI) removal from contaminated water. Plant extracts of Sorbaria sorbifolia were utilized for the synthesis of peCSs via a single-step hydrothermal carbonization, and the physico-chemical properties of the synthesized peCSs were characterized by spectroscopic analysis. The peCSs showed high nitrogen content (∼7.49%) due to the presence of naturally occurring cyanogenic glycosides and mesoporosity (8.31 nm). The plant extract concentration played an important role in determining the size of the peCSs, which ranged from 0.5 to 3.0 μm. The adsorption capacity (Qm) of peCSs for U(VI) (Q_m ≈ 113 mg g⁻¹) was higher than that of the glucose-derived CSs (Q_m ≈ 57 mg g⁻¹) and commercial powdered activated carbon (Q_m ≈ 44 mg g⁻¹). A plausible mechanism for the higher adsorption efficacy of peCSs was proposed via sorbate–sorbent interactions. The ionic strength (0.01 M to 1 M NaCl) showed the weakest effect on the U(VI) adsorption. The multiple adsorption–desorption cycling test revealed that the efficacy of peCSs does not significantly decrease after repetitive use.