© 2021 Taylor & Francis Group, LLC.CdSe-carbon nanodots modified magnetic Fe3O4 nanospheres (CdSe-NDs@C-NDs@Fe3O4) were synthesized by a simple hydrothermal technique and characterized by Fourier transform-infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) with mapping, and energy dispersive X-ray spectroscopy (EDX). The prepared material was used as a magnetic solid phase microextraction (MSPE) adsorbent to determine malachite green (MG). Pasteurized cow’s milk was used as the carbon source to fabricate the carbon nanodots. The spectrophotometric determination of malachite green was performed at 625 nm. The parameters affecting the microextraction efficiency were optimized, including the pH (9.0), mass of adsorbent (5 mg), eluent type (0.1 M acetic acid in ethanol), eluent volume (0.75 mL), sample volume (10 mL), in addition to matrix studies. In order to evaluate the accuracy of the method, real samples were subjected to addition-recovery tests and the recovery values from 85% to 104% were obtained. The limits of detection (LOD) and quantitation (LOQ) of the developed method were 0.013 µg mL−1 and 0.042 µg mL−1, respectively.