Tetranychus merganser Boudreaux (Acari: Tetranychidae) is one of the most economically important mite pests of papaya (Carica papaya L.) crops grown along the Gulf of México. Control of this mite depends mainly on chemical insecticides. This research aims to evaluate the effects of different concentrations (0.1, 0.5, 1, 5, 10, and 15% [v/v]) of the ethanolic extract of Magnolia alejandrae García-Morales and Iamonico (Magnoliaceae) leaves on T. merganser adult females, as well as to obtain information about the chemical composition of the extract. The ethanolic extracts contained some phytochemicals such as phenolic compounds, alkaloids, glycosides, ketones, terpenes, and quinones. Females treated with 0.1 and 15% (v/v) of the extract showed mortality of 0.0% and 33.3% at 72 h, respectively, as compared to the control treatment. The mites treated with 15% (v/v) of the extract decreased their oviposition rate (5.90, 5.35, and 4.77 eggs/female), compared to the control treatment (13.27, 13.95, and 14.39 eggs/female) at 24, 48, and 72 h, respectively, which led to a reduction in the growth rate. The feeding damage caused by T. merganser was reduced at high concentrations. Magnolia alejandrae leaf ethanolic extract has potential for development as a control agent for T. merganser.