The use of plants that present clear physiological responses to heavy metals such as cadmium (Cd) can assist in environmental monitoring. The aim was to evaluate physiological responses of Cd in Alternanthera tenella plants in micro-controlled conditions and to assess the its tolerance level and bioindicator potential. Nodal segments of A. tenella were grown in flasks containing semi-solid MS medium supplemented with (0, 50, 100, and 150 μM Cd). The total fresh and dry weight of the plants, in addition to physiological analyses, were determined after 30 days of cultivation. Plants submitted to Cd showed a reduction in biomass and photosynthetic pigments content. The decline in the Chl a/b ratio indicated the deleterious effects of Cd, as evidenced by the decrease in the density of active reaction centers (RC/CSM). The levels of deterioration in the functionality or structural integrity of the thylakoid membranes (WL) and the activity of the oxygen evolution complex (WK) increased as a function of Cd concentrations. The decreases in photochemical and non-photochemical quenching (qP and qN, respectively) observed in plants exposed to Cd indicated photochemical dysfunctions. Additionally, the increase in the quantum yield of non-regulated energy dissipation (ΦNO) indicated the presence of photodamage. Plants had a low/intermediate TI level (≥21.7) in the analyzed concentration range. A. tenella plants can bioaccumulate high amounts of Cd. The level of physiological responses of A. tenella is dependent on the concentration of Cd, which makes the use of this species even more promising as a bioindicator for this metal.