Non-stochiometric copper selenide nanocrystals are p-type semiconductors, which present localized surface plasmon resonances (LSPRs) in the near-infrared region, which are proportional to the Cu(I) vacancies. Here, an easy, low-cost, and environmentally friendly methodology for the one-pot aqueous colloidal synthesis of non-stoichiometric copper selenide nanocrystals (NCs) is described. The developed methodology consists in the use of CuCl and Na₂O₃Se as precursors of copper and selenium, respectively, stabilized by mercaptosuccinic acid (MSA). The best conditions for the non-stoichiometric NCs regarding Cu/stabilizer and Cu/Se molar ratios, and the initial pH, that provided the prominent plasmonic, were those with a Cu/Se molar ratio of 1 : 2 and pH 5.0, and both molar ratios of Cu/MSA used (1 : 6 and 1 : 9) with stirring for 60 min, at 90 °C, and under N₂ flux. On the other hand, the nominal stoichiometric condition (Cu_2−XSe, X = 0) presented a very weak LSPR behavior, suggesting lower copper vacancy defects, as expected. The obtained plasmonic NCs were characterized optically by electron absorption spectroscopy and structurally by X-ray diffractometry, transmission electron microscopy (TEM), Raman, and infrared spectroscopy (FTIR). For the best synthetic condition, the NCs produced presented a maximum absorption band at 1124 nm. TEM images for non-stoichiometric copper selenide systems showed highly crystalline and mostly spheroid particles, and a rather large size dispersion ranging from 2 to 13 nm, with a mean size of d = 8.6 ± 2.0 nm. The crystalline structure determined by the X-ray diffractometry profile for these as-prepared non-stoichiometric nanoparticles matches those observed for the Klockmannite hexagonal structure for CuSe (X = 1) structures. Furthermore, the use of the synthesized Cu_2−XSe-MSA NCs in surface-enhanced infrared absorption (SEIRA) spectroscopy promoted an effective enhancement of infrared signatures of molecules in a very low concentration, as shown in our preliminary results with standard dye molecules.