Penoxsulam, a new post-emergent rice herbicide, is widely used in rice fields in Egypt. Penoxsulam has been shown to have adverse effects on plant growth; hence, it prevents plants from producing an important enzyme, acetolactate synthase. Glutathione-s-transferase (GST) plays a vital role in the detoxification of xenobiotics including Herbicides. Genetically-engineered algae can provide feasible and environmentally safe approaches for the phycoremediation of herbicides. In the current study, Cyanobacterial GST was transferred to Chlamydomonas reinhardtii, a model unicellular alga, via the Agrobacterium tumefaciens-mediated transformation method. The GST enzyme activity was confirmed in three transgenic C. reinhardtii lines. To evaluate the removal capacity of penoxsulam by transgenic C. reinhardtii, the microalgae were treated with penoxsulam at 5, 10, and 20 μg ml-1 for twenty-four hours. The percentage of the herbicide removal from 5 and 20 μg ml-1 reached maximally 93.6 % and 54 % in the case of transgenic alga, whereas in the case of the wild type, the percentage of removal reached 52 % and 21 %, respectively. Langmuir and Freundlich adsorption isotherm models and the sorption kinetics have been applied to the experimental data to check the effectiveness of the removal process. The equilibrium data were well-fitted with the Langmuir isotherm model. The results showed that GST transgenic microalgae are effective in the removal of penoxsulam from aqueous solutions and it can be used in phycoremediation systems of herbicides.