Cu is the cheapest plasmonic metal showing plasmonic resonance in the visible region, which makes it highly attractive in various fields (e.g., sensing, surface-enhanced Raman scattering, and photocatalysis). However, its poor chemical stability severely restricts its application. Herein, we develop a seed-mediated approach to synthesize ultrastable Cu-based nanoparticles (NPs) stabilized with a thin, completely covered shell. By precisely controlling the reaction conditions, we are able to achieve uniform plasmonic Cu–Au core–shell NPs with significantly enhanced chemical stability even in a harsh environment in the presence of a strong oxidizing acid (HNO3) solution. In-depth characterizations and analysis allow us to identify the critical role of the external crystalline Au layer, as compared to the AuCu alloy layer, in achieving superior stability. Furthermore, a deeper understanding of the plasmonic spectra was obtained by correlating the theoretical calculations on NPs of different core–shell dimensions with experimental results. Transient absorption measurements reveal that the plasmon dynamics and the heat transfer coefficients are not affected with the shell formation. As a proof of concept, these NPs demonstrate high photothermal efficiency and chemical stability for solar steam generation. This work offers a general strategy for the synthesis of ultrastable cost-effective, plasmonic Cu-based NPs, which show great potential in catalysis, electronics, and optics.