•High-density coherent nanoparticles are formed in a 3D-printed&aged Cu-based CCA.•The high density is due to the supersaturation state by rapid cooling of 3D printing.•The nanoparticles are formed by spinodal decomposition and thus coherent with matrix.•The CCA is notably strengthened by the nanoparticles without ductility loss. Metallic 3D printing enables fast fabrication of net-shaped components for broad engineering applications, yet it restrains the use of most mechanical processing methods for strengthening alloys, e.g. forging, rolling, etc. Here, we proposed a new strategy for enhancing the strength of 3D printed complex concentrated alloys without losing ductility. This strategy relies on the rapid cooling of 3D printing to achieve a supersaturation state that is beyond conventional casting. Then, spinodal decomposition via aging is exploited to introduce high-density coherent nanoparticles for strengthening. The proposed strategy is demonstrated in a 3D printed Cu-based complex concentrated alloy. The rapid solidification during printing strongly inhibits elemental diffusion, leading to a high supersaturation state. High-density nanoparticles with coherent interface and size of ∼7 nm are introduced into the 3D printed samples through spinodal decomposition via simple aging treatment. The strength of the 3D printed alloy is increased by 30 % after aging with no ductility loss, leading to a strength-ductility combination superior to other Cu alloys. This strategy is readily applicable to other spinodal alloys fabricated by 3D printing for circumventing the strength-ductility trade-off dilemma.