Environmental pollutants, including organic dyes and plastic, have posed severe threats to the health of our ecosystem. Although a lot of efforts have been made to tackle this problem, these pollutants are still massively produced during industrial activities as well as human life. The ability to detect these environmental pollutants with high sensitivity is thus greatly desired. Herein, we propose a novel surface-enhanced Raman spectroscopy (SERS) substrate with high-density hot spots, high Raman signal enhancement, and good uniformity for trace detection of organic pollutants and nanoplastic. The SERS substrates consist of abundant ultranarrow nanogaps that are fabricated with cost-effective colloidal lithography. With the significantly enhanced near-field intensity, rhodamine 6G (R6G) molecules as low as 10⁻⁸ M can be detected with good linear correlations between the signal intensity and molecular concentrations. Moreover, both nanowells and nanogaps are present on the SERS substrates fabricated by our method and the nanowells could be used to trap micro- or nanoplastic. We successfully demonstrate detecting polystyrene (PS) nanospheres (200 nm in diameter) with different concentrations (1.000% to 0.001%), showing the potential to detect micro- or nanoplastic pollutants in the environment. Our method provides a feasible approach to fabricating low-cost SERS substrates for the detection of organic pollutants and nanoplastic.