This study evaluates the chlorophyll index and fluorescence parameters of chlorophyll a in Alternanthera tenella grown in different concentrations of copper (Cu). The A. tenella plants were hydroponically grown in a complete nutrient solution for 3 days and were then transferred to the same solution containing different copper concentrations [0.041, 0.082 (control), 0.164, 0.246, and 0.328 mM]. The plants were maintained in these solutions for 13 days. The nutrient solutions were replaced every 3 days, and the plants were evaluated at 6 days and 13 days after the start of the treatment. An increase in the chlorophyll index was observed at copper concentrations of 0.164, 0.246, and 0.328 mM on days 6 and 13. These concentrations of copper also affected the fluorescence parameters and caused a reduction in the pool of the final photosystem I (PSI) electron acceptors, a decrease in the total number of electron carriers per reaction centre, a decrease in the parameters related to the flow, yield, and efficiency for the reduction of the final PSI acceptor, and a decline of the total photosynthetic performance index. Therefore, high doses of copper affect the functionality of the PSI, decreasing the reduction flow of the final PSI electron acceptors.