An electrochemical sensor based on a CdSe nanoparticles (NPs)-decorated poly(diallyldimethylammonium chloride) (PDDA)-functionalized graphene (CdSe–PDDA-G) nanocomposite was fabricated for the sensitive detection of esculetin. The nanocomposite was characterized by X-ray diffraction (XRD), ultraviolet/visible spectra (UV-vis) and transmission electron microscopy (TEM). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to investigate the electrochemical behaviors of esculetin on the CdSe–PDDA-G composite film-modified glassy carbon electrode (GCE). The experimental results indicated that the incorporation of CdSe NPs with PDDA-G greatly enhanced the electrochemical response of esculetin. This electrochemical sensor displayed satisfactory analytical performance for esculetin detection over a range from 1.0 × 10−8 to 5.0 × 10−5 mol L−1 with a detection limit of 4.0 × 10−9 mol L−1 (S/N = 3). Moreover, the sensor also exhibited good reproducibility and stability, and could be used for the detection of esculetin in real samples with satisfactory results.