Corn silk (CS) modified by dilute nitric acid as a novel biosorbent has been used for the first time for solid phase extraction of trace copper in water samples. Characterizations of the modified CS have been investigated by means of FESEM images, FT-IR spectra and surface charge analysis. A new functional group, i.e., nitric ether (–O–NO₂) is introduced onto the HNO₃-modified CS, which is favorable for the adsorption of Cu²⁺. The abundant negative charges on the surface of the modified CS also provide great potential for the retention of Cu²⁺ on its surface. The maximum retention of Cu²⁺ ions is obtained under the pH range of 4.0–7.0, corresponding to a theoretical maximum adsorption capacity of 95.7 mg g⁻¹. The adsorbed Cu²⁺ is then quantitatively eluted using a 1.5 mol L⁻¹ nitric acid solution with detection by flame atomic absorption spectrometry (FAAS). With a sample volume of 30 mL, an enrichment factor of 39 and a precision of 1.6% RSD (40 μg L⁻¹, n = 11) are achieved. The concentration of Cu²⁺ could be accurately quantified within a linear range of 1–120 μg L⁻¹ and with a detection limit (3σ) of 0.35 μg L⁻¹. The present procedure is validated by analyzing a certified reference material of Trace Elements in Water (GBW(E)080039). It is further applied to determine copper by spiking recovery tests in tap water, spring water, snow meltwater, lake water, river water and an electroplating water sample, giving rise to spiking recoveries in the range of 96.8–103.0%.